Liu Zhongchun Yuan Xiangchun Li Jianglong

(Sinopec Petroleum Exploration and Development Research Institute, Beijing 100083)

Abstract Taha oilfield Ordovician carbonatite slit-hole type thick oil reservoirs are affected by many tectonic movements, karst slit-hole interaction development, the depth of more than 5,300 meters, the oil and water distribution relationship is complex, non-homogeneous and extremely strong. The characteristic scale of flow in reservoir space is as large as tens of meters and as small as micrometers, and the flow law is different from that of sandstone reservoirs. The production dynamics of oil wells are variable, and the controllability of development is poor. In order to y study the residual oil form in carbonate slit-hole type reservoirs, and to reveal whether there is still a value to be utilized after well flooding, based on the comprehensive interpretation data of oil wells, production dynamic information, and combining with the understanding of karst slit-hole and ancient karst slit-hole in modern karst landscape, three kinds of simplified geologic models of near-well zone reservoirs have been set up, and the method combining the theory of hydrodynamics and physical simulation experiments is used to analyze the water flooding of oil wells encountered with different Using a combination of hydrodynamic theory and physical simulation experiments, we analyzed the form of residual oil after water flooding in wells drilled into different reservoir spaces, and established the research direction of enhanced recovery technology for seam-hole carbonate reservoirs.

Keywords: Slit-hole carbonate reservoir, Geological model, Physical simulation, Residual oil form

Analysis on Formation of Residual Oil Existence and Its Effect Factors in The Forth Area of Tahe Carbonate Reservoir

LIU Zhong-chun, YUAN Xiang-chun, LI Jiang-long

(Exploration ＆ Production Research lnstitute, SlNOPEC. Beijing100083)

Abstract In Tahe Ordovician carbonate reservoir, which is karstic/fractured heavy oil reservoir, higher level of heterogeneity and more complex distributing of oil and water had been formed by ancient structural action time after time comparing with other carbonate The reservoir depth is over 5300m and temperature is 398K.The oil viscosity is about 24mPa-s on the reservoir condition.The main flow conduits include fractures and caves. The main flow conduits include fractures and caves that their flow characteristic sizes are from several decameters to microns. The well production performances vary rulelessly, and are difficult to be controlled.For investing the form of residual oil existence and analyzing the value in use of the well after water For investing the form of residual oil existence and analyzing the value in use of the well after water out, three types of simplified theorial and experimental models were constructed separately combining the results of integrated interpreting and production performance information of wells with realization production performance information of wells with realization of modern and ancient karst.As to the wells drilling on different flow conduits in carbonate reservoirs, the form of the wells was analyzed and analyzed the value in use of the well after waterout. As to the wells drilling on different flow conduits in carbonate reservoirs, the form of residual oil existence and its effect factors have been discussed. Meanwhile, the direction of EOR technology development in fractured/karstic carbonate reservoir have been determined.

Key words Fractured/karstic carbonate reservoir Theoretical model Physical simulation Form of residual oil

Carbonate oil and gas fields occupy an important position in the world's oil and gas distribution, and their reserves account for more than 50% of the total oil and gas reserves, while the production has accounted for about 60% of the total production [1, 2]. In recent years, the exploration and development of carbonate oil and gas fields in China has also shown a rapid development trend, especially the rapid development of the Tarim Basin's Tahe Oilfield. By the end of 2005, the cumulative proven oil geological reserves of the Tahe Oilfield amounted to 6.3×108t, with an annual oil production of 4.2×106t, which has become the largest Paleozoic carbonate oilfield in China. The Ordovician oil reservoir in Area 4 of the Tahe Oilfield is located in the center of the Tahe Oilfield, with the Aixieke No.2 structure as the main body, and is a carbonate rock karst slit hole-type massive heavy oil reservoir with bottom water. The depth of the reservoir is more than 5300m, and the reservoir type is dominated by caves, and the development is extremely irregular, with strong longitudinal and transverse non-homogeneity, which makes the reservoir prediction difficult, and the relationship between oil, gas and water and the reservoir type is extremely complicated. After nearly 10 years of rolling exploration and development, the development characteristics of low drilling success rate, low recovery rate and fast decline have been exposed. The oil wells see water too early, natural energy is insufficient, and the water content rises quickly; the fastest annual decline rate of the reservoir is as high as 44%, and violent water flooding can sharply reduce the production of the oil wells by more than 70%; the degree of utilization of planar and longitudinal reserves is low, and the average degree of recovery is only 9.5% [5~11]. Therefore, on the basis of the existing geological understanding of the reservoir, it is imminent to study the residual oil form of seam-hole carbonate reservoirs and explore new methods to improve the recovery rate.

1 Recognition of caverns, seams and matrix blocks in cavernous carbonate reservoirs

Logging, drilling, logging and the production dynamics of the wells show that some wells are directly drilled into unfilled or semi-filled caverns, and production is built directly; some wells are not directly drilled into the caverns, but the acid pressure can communicate with the space of the effective reservoir; and there are also a few wells drilled into the dense rock, even if acid pressure can not communicate with the effective reservoir capacity; and there are also a few wells drilled in dense rock. There are also a few wells drilled in dense rock, even if acid pressure can not communicate effective reservoir space. Recognizing the reservoir characteristics of cavern type reservoirs, identifying the distribution of effective reservoir space, and understanding the distribution pattern of residual oil are the basis for improving the recovery rate of the reservoir.

1.1 Awareness of caverns

Theoretically, the underground paleokarst cavern characteristics should have some similarity with modern karst. Figures 1 and 2 show the distribution and cave chamber of the world's longest modern karst Shuanghe Cave in Guiyang, China.

Figure 1 Plane distribution of Shuanghe Cave

Figure 2 One of the cave chambers in Shuanghe Cave

Modern karst development is characterized by the following features: (1) cave spreading is controlled by regional tectonic fissures; (2) cave development is closely related to the underground drainage system; (3) multi-phase karst action has formed the cavern with multi-layer; (4) erosion and deposition of the cavern are synchronous; (5) the cavern is developed in the core and near flanks of fold; and (6) the cavern is developed in the core and near flanks of the fold; and (7) the cavern is developed in the core and near flank of the fold. core and near the wing; ⑥ large caves are mostly located in the middle and upper reaches of the river; ⑦ underground river as the main body, the development of a number of branch caves; ⑧ cave scale is large, the longest up to 85.3km (Shuanghe Cave); the largest cave area of × 104m2 (Zijin Cave), up to 150m.

Palaeokarst system, due to long-term tectonic movement and depositional effects, the key layer of the overburden rock layer due to the rock self-weight, gravity, and the concentration of ground stress, as well as solvents, the key layer of the rock mass. Gravity, geostress concentration and the vacuum negative pressure inside the cavern destroyed and collapsed. The serious voiding and leakage phenomena in some wells during the drilling process of Tahe Zone 4 fully indicate the existence of unfilled caverns. However, the results of logging interpretation show that most of the karst systems are filled to varying degrees, such as T403 wells with fully filled holes up to 67 m, and TK409 wells with fully filled holes up to 75 m. Fig. 3 shows the results of comparison between logging and imaging logging of TK429 wells, with a depth of 5420.0-5427.5 m and a thickness of 7.5 m, which is a section of cave development. There are collapsed breccia, dark river sedimentary breccia and sandy mudstone deposits, and dense chert in the large cavern (Figure 4).

The main difference between the paleokarst system and modern karst is that the size of the caves is smaller than the ground and the caves are highly filled.

Figure 3 Logging and imaging of KT429 well

Figure 4 Different kinds of fillings in the cave

1.2 Distribution of fracture development

According to the data of 14 imaging logs in the Taha Oilfield, the direction of the fractures was counted, and the results are shown in Figure 5, which shows that the NW-SE direction of the fracture system dominates the fracture system of the region, and the NW-SE direction is the most dominant in the fracture system. It can be seen that the NW-SE fracture system is dominated by the NW-SE fracture system, in which the fractures with a strike of 160°～180° or 350°～360° are dominated, and the development of the NE-SW fracture system is significantly worse than that of the former one, which has a main strike of 0～20° or 180°～220°. The inclination angles of the cracks are shown in Figure 6. Most of the cracks have dip angles in the range of 60° to 90°, and most of the cracks are produced at high angles, with few low-angle cracks developed. Most of the effective cracks in the Ordovician carbonate rocks are mainly concentrated in the karst section where the slip and collapse breccia phenomenon exists locally, so the cracks are mainly related to the karst collapse in terms of genesis.

Figure 5 Overall strike characteristics of the Ordovician fracture system in the Taha Oilfield

Figure 6 Percentage of fracture inclination

1.3 Recognition of the matrix block system

According to the pore-permeability analysis of the core of the Lower Ordovician reservoirs, the porosity distribution of the small samples of Block 7011 is from 0.01% to 10.8%, with an average of 0.96%, and samples with porosity less than 1% account for 71.52% of the total. The average is 0.96%, of which samples smaller than 1% account for 71.52%, 1.0% to 2.0% (including 1.0%) account for 22.02%, and those larger than 2% only account for 6.46%. The permeability distribution of 6473 small samples in the region was (0.001～5052)×10-3μm2, of which 67.14% of the total number of samples was less than 0.12×10-3μm2, 85.68% was less than 0.6×10-3μm2, 94.39% was less than 3×10-3μm2, and only 5.61% was greater than 3×10-3μm2.

2 Simplified geological models and residual oil in the near-well zone

In order to further reveal the relationship between the production dynamics of oil wells and the nature of reservoirs, and whether there is still value to be utilized and the form of residual oil after flooding of the oil wells, four different geological models of the near-well zone were established based on the analysis of the comprehensive data of the oil wells.

2.1 Closed-type cavern

Closed-type pure oil cavern refers to the cavern that does not communicate with the outside world, and the interior is only filled with oil. This type of cave has not yet been found to be drilled, but there is not yet sufficient evidence to rule out the possibility of its existence.

This type of cavern relies entirely on natural elastic energy, including the elastic energy of crude oil and the elastic energy of the cavern cracks themselves. As there is no external energy supplement, the pressure inside the cavern and the production rate of the production well are gradually reduced due to the loss of natural energy, until the final spraying stops.

2.1.1 Analysis of residual oil using the material balance method

Producing wells drilled into such caverns stop spudding when the bottomhole flow pressure is lower than the static column pressure of the wellbore and the pressure loss due to wellbore friction.

pwf=Δp(static liquid column)+Δp(friction) (1)

For wells with barehole completions, the pressure inside the cavern is close to the value expressed in equation (1) when the blowout stops, and at this time, according to the equation for the balance of matter, the cumulative oil recovery of the well is:

NpBo=NoBoCt(pi-pwf) (2)

The recovery rate of this type of well is only related to the pressure inside the cavern, which is the same as that inside the well, but it is also related to the pressure loss of the wellbore. The recovery rate is only related to the elastic compression coefficient and pressure drop of the crude oil and rock in the cavern, which is in accordance with the following formula:

Theory of oil and gas reservoirs and exploration and development technology

Whether the wellhead restricts the production or not, there will be residual oil in the wells drilled in the cavern at any location, and the size of the residual oil meets:

Residual oil = (1-η)NoBo (4)

2.1.2 Flow characteristics of fluid in the cavern

Based on Bernoulli's equation in fluid dynamics

Oil and gas reservoir theory and exploration and development technology

the flow characteristics of single-phase fluid in the cylindrical cavern were calculated, and the results of the acausal distribution of the pressure and flow rate are shown in Fig. 7. When the closed cavern with a certain pressure is opened, the flow line of fluid in the cavern is shown in Fig. 7. The pressure is perturbed only in the near-well zone; away from the bottom of the well, the pressure remains in the initial state. The flow rate of the fluid begins to perturb at an acausal distance of 0.5 m, i.e., near one-half the height of the cavern.

Figure 7 Flow characteristics of single-phase fluid in a single well of a cylindrical cavern

2.2 Bottom-water caverns

Bottom-water caverns are subdivided into closed bottom-water caverns and communicating bottom-water caverns. Among them, closed bottom-water caves are caves that do not communicate with the outside world and include both oil and water phases inside (Figure 8). This type of cavern also relies entirely on natural elastic energy for mining, which includes the elastic energy of crude oil, stratum water and the elastic energy of the cavern cracks themselves. Communication-type bottom water cavern refers to the communication with the outside world, and can be divided into two kinds, one is the outside world water flooding rate is lower than the production rate, at this time the cavern relies on the natural energy, including the amount of water flooding and elastic energy; another is the outside world water flooding rate is equal to the production rate, the cavern pressure is unchanged, the exploitation of this type of cavern relies entirely on the water drive.

2.2.1 Theoretical analysis of the bottom water cone in unfilled caverns

For bottom water type caverns, the reason for the decreasing production of wells is not only the decrease of energy, but also the effect of water discharge. Water out of the oil well accelerates the decreasing production. Water out of the oil well does not mean that the oil-water interface must reach the bottom of the well, according to the theory of fluid mechanics, the velocity of oil and water at the oil-water interface are:

Oil and gas reservoir theory and exploration and development technology

Oil and gas reservoir theory and exploration and development technology

Water-oil velocity ratio:

Oil and gas reservoir theory and exploration and development technology

Taha River oil field 4 area Underground crude oil viscosity is 24 mPa-s on average, and if the formation water viscosity is approximated to 1 mPa-s, then the water velocity is 24 times the oil phase velocity under the same conditions. Therefore, when the cavern is drilled open, due to the disturbance generated by the production well, the tendency of bottom water coning in near the bottom of the well is bound to occur, while the gravity separation effect caused by the difference between the density of oil and water can inhibit the bottom water coning in.

Figure 8 Schematic diagram of closed bottom water cavern

The residual oil in this type of cavern not only depends on the natural energy in the cavern, but also is closely related to the degree of bottom water coning. The breakthrough of bottom water from the production wells accelerates the process of stopping the oil wells. Therefore, the factors affecting the degree of bottom water coning will also affect the amount of remaining oil in the cavern. There are a number of factors that influence this, including the viscosity ratio of oil and water, the strength of the oil recovery, the height of the oil-water interface in the cavern, the location of the production wells, the density of the production wells, and the geometry of the cavern.

Figure 9 Experimental results of bottom water coning

2.2.2 Physical simulation of bottom water coning in unfilled caverns

The experiment uses a vacuum pump to generate negative pressure flow to simulate the process of bottom water coning in cavern-type reservoir spaces. The oil used for the experiment is white oil with a viscosity of about 15mPa-s, and the water is brine with a mineralization of 2×105mg/L. The temperature of the experiment is 25℃ at room temperature, and the results of the experiment are shown in Fig. 9.

The displacement rate of the experiment is 30mL/s, i.e., 2.5t/d, and the height of the water cone produced is about 0.01m; the reduction of the production rate can suppress the production of the water cone; the perturbation of the water cone in the bottomhole is very small. The range is very small. Due to the result of gravitational differentiation of oil and water, the actual water cone height is much smaller than the result of theoretical calculation. If we assume that the height of the water cone is proportional to the production rate, then it is estimated that when the actual production rate reaches 250t/d, the height of the water cone is only 1 m. Therefore, it can be assumed that when the well is at the top of the unfilled cavern, the potential of the residual oil is very small after the well has seen the water and this part of the residual oil can be extracted efficiently by reducing the production rate.

2.3 Near-well Seam Hole Type

The wells drilled in Tahe Oilfield Area 4 that encountered the cavern and finalized the hole in advance were a minority after all, and most of the wells completed the drilling process normally, with some of the wells built up to production after natural completion, and some of the wells built up to production after acid pressure. The results of core observation and imaging logging interpretation have a certain degree of understanding of the seam hole drilled in the bare borehole section.

Figure 10 Cavities and seams drilled in bare eye well section and simplified model

For the theoretical study, the cavities and fractures drilled in bare eye well section are simplified into a set of regular capillary flow (Figure 10). According to the statistical results of core observation, there are 19 cracks with a width greater than 1mm, accounting for 2.4% of the total; 267 cracks*** with a width of 0.1-1mm, accounting for 33.5% of the total; and 512 cracks*** with a width of less than 0.1mm, accounting for 64.2% of the total.

Based on the theory of fluid dynamics, according to the ratio of seams counted in the core, different scales of seams and holes contribute differently to the total flow rate into the bare eye well section. The results show that when there is a hole, even if there is only one, when the scale of the hole is large to a certain extent, such as when the scale of the hole is larger than 50mm, the contribution to the total flow has been greater than 95.96%. That is to say, when the scale of the hole is larger than 50mm, the total production of the well mainly comes from the hole, and the contribution of the seam is smaller. The main forms of residual oil include residual oil in the seam that is not rippled by the bottom water, rippled through the wall of the oversized hole, and the quantity depends on the degree of non-homogeneity and the viscosity ratio of oil and water.

According to the above size and ratio of the hole and slit, the proportion distribution of the hole and slit reserves in the near-well zone is shown in Fig. 11. When the scale of the hole is 1m, the reserves in the hole account for 82% of the total reserves, and the reserves in the slit only account for 17.8%; when the scale of the hole is reduced to 50mm, the proportion of the hole reserves to the total reserves is reduced to 18.7%, and the reserves in the slit increase to 81.3%. Although the contribution of the hole to the total flow is still higher when the scale of the hole decreases to 50mm in the bare eye well section, the storage in the seam cannot be neglected after the fluid in the hole is replaced by the bottom water.

Figure 11 Percentage of reservoir volume accounted for by different scales of caverns per unit rock volume

2.4 Near-well fracture type

Most of the wells in Area 4 of the Taha Oilfield are constructed and produced after acid pressure, i.e., the wells were not drilled to encounter the effective reservoir space during the drilling process, and then communicated with the effective reservoir space to construct and produce after acid pressure (Fig. 12). For the convenience of the study is still simplified as a bundle of capillary.

Figure 12 Bare-bore well section drilled cracks and simplified model

Since the surface of the carbonate rock is oleophilic, when the bottom water drives the crude oil in the cracks, the force of the capillary is the resistance to drive, and a residual film of oil is inevitably left in the wall of the cracks. Comparison of the hydrophilic and oleophilic pore space in the water drive process is shown in Fig. 13.

Fig. 13 Distribution of oil and water in different wettability simulation pore model

Still according to the above analyzed fracture distribution ratio, the residual oil percentage for different thickness of the film is shown in Fig. 14.

It can be seen that, for a certain volume of the fracture reservoir, assuming that the bottom water coverage reaches 100 percent, according to the calculation of residual oil film with different thicknesses, the residual oil will be removed when the oil film reaches the bottom water coverage. It can be seen that for a certain volume of fracture reservoir space, assuming that the bottom water coverage reaches 100%, only according to different thicknesses of residual oil film, when the thickness of oil film reaches 0.1mm, the percentage of residual oil is close to 50%, and when the thickness of oil film decreases to 0.01mm, the percentage of residual oil can reach 26%. The thickness of the oil film is not only related to the wettability of the rock, but also depends on the replacement rate. Moreover, the bottom water can't drive out 100 percent of the fracture pore space, so the remaining oil in the fracture-type reservoir space is also considerable.

Figure 14 Percentage of residual oil with different film thickness

3 Residual oil generating factors and ways to improve recovery

According to the residual oil analysis of the geologic model, the key problems of improving recovery in fracture pore carbonate reservoirs at present are: (1) wells fail to communicate effectively with the effective reservoir space; (2) even if wells communicate with the effective reservoir space, a large amount of oil can still be generated due to the cone of bottom water or the insufficient natural Even if the wells communicate the effective reservoir space, a large amount of residual oil can still be generated due to bottom water coning or natural energy shortage. For the utilized reserves, the influencing factors of residual oil in bottom water carbonate reservoirs include energy and the degree of subsoil water replacement, and the degree of subsoil water replacement can be analyzed from the perspective of sweeping efficiency and washing efficiency, and the results are shown in Fig. 15.The size of the natural energy of the reservoir, the degree of non-homogeneity, and the viscosity ratio of the oil and water are the three key factors influencing the recovery rate of utilized reserves in the seam-hole type carbonate reservoirs.

Figure 15 Factors affecting the recovery rate of seam-hole reservoirs and ways to improve the recovery rate

Therefore, for this kind of reservoirs, combined with the analysis of residual oil morphology, we should carry out targeted research on the technology to improve the recovery rate. To "overall control of water pressure cone, improve the well plane and longitudinal reserve utilization capacity" as the immediate goal, "replenishment of energy" and other methods to improve recovery as a follow-up guarantee of the research work is imperative. Specifically, it can be divided into two phases: the first is the natural energy phase, including encrypted wells, vertical layered mining, side drilling horizontal wells, acid pressure, water plugging and other technical research; the second is the artificial energy supplementation phase, the possible methods include water injection, gas injection, thickening agent injection, and active agents and so on. The chemical method is more risky; although gas injection has a unique advantage for reservoirs with bottom water and vertical fractures, injection pumps requiring higher injection pressure for reservoirs buried deeper than 5300m limit the application of this method. Therefore, water injection is still the preferred method with low risk and low cost. However, the successful water injection experience in conventional reservoirs is no longer adapted to slit-hole carbonate reservoirs where connectivity cannot be judged [3, 4], so the study of new and effective water injection methods is imminent.

4 Conclusion and Recognition

(1) Water flooding of oil wells only indicates water flooding of the large oil outlet channels, which does not mean that the reservoir space is completely flooded.

(2) The main residual oil is mainly in five forms: ① residual oil not covered by the bottom water due to the difference in the scale of the reservoir space; ② residual oil on the top of the oil wells not located in the top of the hole, and the top of the hole is not filled with the cave after the flooding; ③ residual oil in the unfilled cave due to the cone of the bottom water; ④ residual oil film after the water has been affected by the flooding; and ⑤ residual oil in the various types of reservoir spaces that are seriously insufficient in terms of energy.

(3) The research on recovery enhancement technology should focus on different types of residual oil, based on the flow unit of the cavern, and determine "overall water control and pressure cone, improve the ability to utilize the reserves in the plane and longitudinal direction of the wells" as the near-term goal, and "replenish the energy" as the goal to enhance recovery. The research direction of the enhanced recovery method is determined based on "overall water control and pressure cone, improve reservoir utilization capacity in the plane and longitudinal direction of oil wells" as the immediate goal, and "replenishment energy" and other enhanced recovery methods as the subsequent guaranteed enhanced recovery methods.

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