Justifying Method of Enhancing Oil Recovery of Multi-zone Reservoirs Including Hydrodynamic Connected Reservoirs

Objectives: This research deals with field development of multi-zone reservoirs including two or more connected reservoirs differing greatly in their reservoir parameters. Development of such fields is characterized by uneven recovery of reservoir reserves. Methods: Recovery efficiency for low-permeability reservoir decreases drastically, in the case of water breakthrough to the bottom hole of the production well through a high-permeable reservoir and a significant part of the reserves is not drained. The key reason is existence of inter-reservoir cross flows arising due to high pressure differentials between the reservoirs. For the purpose of enhancing recovery efficiency of multi-zone reservoirs, modern methods of enhancing oil recovery may be considered. Findings: At the same time, most of the methods do not solve the problem of inter-reservoir cross flows efficiently, thus, providing no increase in drainage of the low-permeability reservoir. The authors provided a new approach based on the application of technology of dual production and injection of water containing suspended particles. This approach ensures a significant reduction in volumes of inter-reservoir cross flows, owing to the use of selective impact of water containing mechanical suspended particles in filtration of reservoirs that differ in reservoir parameters. The composition, size and concentration of water particles are determined at the stage of laboratory tests, using core samples taken from the productive reservoirs. As a result of 3D hydrodynamic simulation of this approach, using sector and full-scale simulation models, a possible increase in the forecasted oil recovery in a wide range of reservoir properties was shown, factors that affect method efficiency were determined. Applications/Improvements: This approach may be applied to enhance oil recovery of multi-zone reservoirs including hydrodynamic connected reservoirs.


Introduction
At the moment, investment into gas and oil industry decrease significantly due to a drop in hydrocarbon prices, thus leading to shortened number of implemented projects in field development and processing and petrochemical industry 1 .The decrease in oil prices affected mostly new projects that have not been launched yet.Many such projects were frozen or closed.Disinvestment in geological field development leads to gradual reduction in reserves of gas and oil companies, as long as oil production rates still increase or remain the same, and reserves, as a result of discovery of new fields or further drilling of existing assets, is not replenished.
Therefore, more and more gas and oil companies pay great attention to methods of enhancing oil recovery, which may be applied to the existing assets to improve their efficiency without the need for essential capital investment.Moreover, today's reserves that may be produced with the application of oil recovery enhancement techniques exceed potential reserves that are still not discovered 2 .
There are a variety of gas and oil methods that involve various mechanisms to enhance oil recovery, including temperature increase of an injection fluid or raising viscosity of such injection fluid, using chemical additives 3 .Efficiency of oil recovery enhancing methods depends on a great number of parameters, including geological and physical properties 2 Justifying Method of Enhancing Oil Recovery of Multi-zone Reservoirs Including Hydrodynamic Connected Reservoirs of the field and pressure and temperature conditions of hydrocarbon reservoirs 4,5 .However, there are such deposits, where these methods may prove low-effective.They include multi-zone reservoirs, involving two or more connected productive reservoirs, differing greatly in their reservoir properties.Water flooding is the most popular technique for enhancing oil recovery in most fields, including multizone reservoirs.When water is injected into a formation, two important aspects of field development are achieved: maintenance of reservoir pressure and oil displacement to the bottom hole of production wells 6 .Water flooding has a lot of advantages, primarily the following: • relatively easy implementation; • easily available injection agents; • relatively high efficiency.
Water flooding is used to develop carbonate, as well as terrigenous reservoirs, excluding any significant limitations on use at high reservoir pressures and temperatures.Speaking of multi-zone reservoirs, where reservoirs are connected and differ significantly in their reservoir properties (10 or more times in their permeability), the injected agent washes reservoir sections with low flow coefficient more intensely, while the efficiency of recovery of reserves of the low-permeability reservoir reduces manyfold following the breakthrough 7 .The reason for such low drainage of low-permeability reservoirs is determined by the presence of inter-reservoir cross flows arising predominantly in the areas of producers and injectors.Draining of reserves of the low-permeability reservoir almost stops, following the breakthrough of the displacement front across the highpermeability reservoir [8][9][10] , due to high water saturation areas near the wells (Figure 1).Inter-reservoir cross flows in the area of injectors are observed even against resisting gravitational forces, i.e. when the high-permeability reservoir is located above the low-permeability reservoir, as long as pressure differential between the reservoirs is too high at significant difference of permeability of productive reservoirs.

Concept Headings
For enhancing production efficiency of such fields, technological solutions to provide for separate production of reservoirs are required.
Downhole configurations for dual production, physical and chemical reservoir isolation methods and technologies aiming at alignment of displacement front may be used.
Dual completion technologies provide for independent production of two or even more reservoirs, using special downhole equipment 11 .Aside from rise in price and difficulty of operation of such equipment, inability to isolate reservoirs in the inter-reservoir space, and, accordingly, to solve the problem of cross flows, and low recovery of reserves of the low-permeability reservoir is the principal disadvantage of the technology for connected reservoirs 10 .However, application of the dual production technology ensures maximum flexibility of the development system for any further geological and technical measures to act on reservoirs separately 10 .
Most physical and chemical methods imply injection of water solutions that contain polymer composites, crosslinkers etc. into the reservoir 12,13 .At the same time, some of these methods aim at isolating reservoirs one from another, while other methods ensure aligning of the displacement front in water flooding.Contrary to the dual production technology, physical and chemical methods ensure reservoir isolation at a certain distance from the well, which allows for reducing or even eliminating any cross flows at all 14 .Main disadvantages of these methods include their high cost, limited application at high pressures and temperatures and exposure to mechanical and chemical destructions, thus specifying low effect duration and the necessity for repeated treatments. 13herefore, for resolving the problem of inter-reservoir cross flows successfully, an approach is required to ensure reservoir isolation not only in the wellbore area, but also at a significant distance from the well for a long time and in the broad range of reservoir temperatures and pressures.
Gas and oil companies conduct a great set of tests aiming at mitigating uncertainties associated with geological aspects, reservoir properties etc. at earlier stages of field production design.Particularly, great attention is paid to laboratory tests of a core, where some tests aim at determining requirements to agents that are injected in the reservoir, aside from such standard parameters as porosity, permeability etc. Allowable concentration, composition and size of suspended particles that may be filtrated freely in the reservoir (including absence of any chemical reactions) are determined in the course of such experiments.Failure to satisfy quality requirements to the injected agent may lead to worsened permeability of the wellbore area of the reservoir and reduced injectivity during field development.Clogging of reservoir pores with fine particles is the main reason for decreased permeability of the wellbore area 15,16 , despite the fact that there is a number of factors 17,18 that affect reservoir parameters of the reservoir in injection.
Mechanism of alteration of permeability of the wellbore area for injecting agents that contain suspended particles relies on particles depositing in pores, and, consequently, reducing cross-sectional area of filtration channels; if the given size of particles is exceeded, complete channel blockage is possible (Figure 2) 19 .

Figure 2.
Processes reducing permeability for injection of suspended particles containing water into a reservoir: 1 -formation of a cake on the outer sample wall and clogging of a filtration channel, 2 -suspended particles depositing and accumulating inside reservoir pores with further reduction of the area that is available for filtration.
Ranges of sizes of particles that are contained in the water solution and determine a specific mechanism of permeability reduction are provided by Bennion and coauthors 19 .The problem of clogging of filtration channels arises, when the average particle size exceeds 1/3• , where k is core sample permeability.If the size of particles that are contained in the water solution is 1/3• to 1/7• , processes of reducing permeability take place directly inside the sample due to deposition and accumulation of the suspended particles. 19ynamics of reduction in permeability that was obtained, for example, in the course of experiments, using core samples from the Berea sand stone, proves the possibility of insignificant reduction in permeability (ca.20-30% for pumping 50 pore volumes), when water that contains particles up to 1/5 in size is injected (Figure 3).

Figure 3.
Dynamics of reduction in permeability for pumping water that contains suspended particles through a core sample (K d /K 0 -ratio of current permeability of the core sample to the initial permeability, d-diameter of suspended particles) 20 .
At the same time, for water containing suspended particles over 1/3• in size, different dynamics of reduction in permeability is typical.When initial pore volumes of suspended particles containing water are pumped, a drastic reduction in permeability is observed, which is indicative of cake formation on the outer edge of the core sample; at the same time, most suspended particles are not filtered inside the sample (Figure 4).For the purpose of solving the problem of isolation of connected reservoirs that differ significantly in their permeability, the possibility of selective impact of suspended particles containing water is suggested.
The suggested solution relies on isolating reservoirs one from another by injecting suspended particles containing water into the high-permeability reservoir, which leads to a gradual reduction in permeability of this reservoir, and, for filtration into the low-permeability reservoir, leads to filtration channels blocked on the border between the reservoirs.This approach may be implemented for high reservoir pressures and temperatures, and it is suitable for terrigeneous and carbonate reservoirs.Tolerance of high pressure and temperature by the suspended particles reduces a great number of repeated treatments of the reservoir, which is a benefit of this approach, as compared to the available physical and chemical methods.This solution will ensure reservoir isolation not only in the wellbore area of the reservoir, but also in the interreservoir area, which, will lead, in its turn, to increased completion of the low-permeability reservoir.From the technical point of view, implementation of this approach is possible with the help of dual completion technology, allowing for action on each reservoir separately.
Prior to reservoir isolation measures, using this method, laboratory testing of core samples is required.Main goal of such testing is determining the right size, composition of particles and their concentration in the solution for gradual creation of additional filtration resistances in the high-permeability reservoir (permeability decrease to 40-80%), and complete clogging of pore channels must be prevented at the same time, even if 100-200 pore volumes are pumped.The availability of the required concentration of particles for complete plugging of pore channels (cake formation) of the core sample taken from the low-permeability reservoir is the absolute requirement to the quality of pumped water.This research does not require any significant time or financial cost and may be done at the design stage of filed developed, provided core material is available.No chemical reactions that lead to any decrease in permeability of the high-permeability reservoir, e.g.due to clay swelling, is another condition of injecting water solution containing suspended particles.
The suggested approach to reservoir isolation should be implemented in several stages (Figure 5).Pumping of treated water that is capable of free filtration in both reservoirs is done at the first stage.When high water content of the product (75-85%) is achieved, a transition to injection of suspended particles containing water into the highpermeability reservoir is suggested, while injection into the low-permeability reservoir should be suspended.Time required for reservoir isolation one from another depends on the capability of suspended particles to plug up filtration channels at the border of two reservoirs.Resuming of injection of treated water into the low-permeability reservoir is suggested, after reservoir isolation is completed.

Results
For the purpose of estimating the efficiency of the suggested technology of reservoir isolation, multi-optional calculations for a 3D hydrodynamic sector model of a symmetry element of 5-point well pattern, including injector and producer in corners of the models were done (Figure 6).Dependence of additional cumulative oil production on the ratio of thicknesses of productive reservoirs for options (Khp/Klp -ratio of permeabilities of productive reservoirs).Dependence of additional cumulative oil production on the ratio of thicknesses of productive reservoirs for options (Khp/Klp -ratio of permeabilities of productive reservoirs).For real fields, dynamics and degree of the reservoir damage formation will also depend on such factors as areal and reservoir heterogeneity of the reservoir parameters, lithologic continuity of reservoirs, existence or absence of impermeable barriers between the reservoirs, uneven pressure distribution in the reservoir, etc.

Simulating the Technology using a Fullscale Model
Following multi-optional calculations based on the sector model, calculation was made, using a full-scale simulation model of a field, characterized by high degree of heterogeneity of the reservoir parameters (Figure 10).Two connected reservoirs that differ significantly in their permeability may be distinguished by the field (Table 1).The basic option suggests field development by means of a single well pattern, using water flooding and simultaneous completion of all reservoirs.According to the simulation, zones of high oil saturation that are not drained remain in a low-permeability reservoir in the basic option (Figure 11).A technology of isolating reservoirs by injecting suspended particles containing water through injectors, which can plug up pore throats of the low-permeability reservoir and gradually reduce reservoir properties of the high-permeability reservoir, was simulated in option 1. Injection of suspended particles containing water starts after water content reaches 80% and continues for a year.After the isolation zone is created between the reservoirs, injection of treated water (i.e.water that may not worsen reservoir parameters of both reservoirs) is resumed into both reservoirs.Comparison of calculation results for various options is given in Figure 12.

Discussion
According to our simulation, most of technological implementation options are known for improved rated cumulative oil production vs. basic values.Additional oil production is ensured due to increased lowpermeability reservoir at water flooding.Increased ratio of permeabilities and increased thickness of the lowpermeability reservoir leads to improved technology efficiency.The highest additional oil production vs. the basic scenario is achieved in options with the highest HP and LP permeability ratio.This is explained by the lowest involvement of the low-permeability reservoir into production, as compared to other options discussed, i.e. the higher difference of productive reservoirs in permeability, the biggest effect of implementation of reservoir isolation technology with other factors being the same.
Implementation of this technology at the earliest development stages may lead to a decrease in reserve recovery of the high permeable reservoir, therefore creating a risk (due to wrong composition, size and concentration of particles selected) of obtaining a negative result of application.Therefore, the isolation technology should be implemented at later stages of field development, when water content of the product is 75-85% and reservoirs are well studied.
Creation of the isolation reservoir allows for reducing inter-reservoir cross flows.The rate of isolation reservoir formation decreases, as the distance from injectors increases, as long as the number of pumped pore volumes per volume of pore space for a given time is reduced.At the same time, the rate of isolation reservoir formation depends on the capability of selected suspended particles to plug up pore throats of the low-permeability reservoir.Calculation results, using sector 3D simulation models, matched highly to laboratory test results, using a core 21 , where the area and degree of reservoir formation damage increases with time.

Conclusions
This approach to development of reservoirs comprising connected reservoirs allows for complete or partial isolation of reservoirs and thus solving the problem of inter-reservoir cross flows.Creation of the formation isolation reservoir ensures increased completion of the low-permeability reservoir drainage in water flooding and, accordingly, increased cumulative oil production.Relaxation in the requirements to the quality of flood water at the time of creation of the low-permeability screen between the reservoirs allows for reduction in water treatment costs.The investigated method of enhancing oil recovery is proposed for use at the later stage of development, when water content of the product is at least 75-85%, for the purpose of reducing any possible effect on recovery of residual reserves of the Justifying Method of Enhancing Oil Recovery of Multi-zone Reservoirs Including Hydrodynamic Connected Reservoirs high-permeability reservoir due to decrease in injection capacity.This method may be applied in the wide range of reservoir temperatures and pressures.Method efficiency depends on a number of factors, including the ratio of reservoir permeability, water content of the product and ratio of thicknesses of productive reservoirs.Successful implementation of this technology requires early laboratory testing, using core samples taken from each productive reservoir.

Acknowledgement
Authors are most grateful and wish to thank Ministry of Education and Science of the Russian Federation (unique project identifier RFMEFI60714X0080) for providing with the funds for completing the research work.

Figure 1 .
Figure 1.Production of reserves of connected reservoirs that differ significantly in their permeability 7 .

Figure 4 .
Figure 4. Change of permeability subject to quantity of pumped pore volumes for low-permeability samples (Kd/ K0-ratio of current permeability of the core sample to the initial permeability, d-diameter of suspended particles).

Figure 6 .
Figure 6.Sector model of an element of a 5-point system of well layout (permeability distribution).This model considers a high-permeability (HP) and low-permeability (LP) reservoirs that exclude any geological barriers to ensure their isolation, i.e. these reservoirs are hydrodynamically connected in vertical direction.The use of dual production technology is considered for the model, and each reservoir is penetrated by an injector and producer.Dual production is simulated by setting two wells, having the same coordinates in the X and Y directions, and varying in perforated intervals in the Z direction.Water-containing suspended particles are injected for a year, and any permeability change is estimated daily.After a year-long injection of suspended particles containing water, water injection into the low-permeability reservoir is resumed.Efficiency of the suggested technology for 25 years of production was estimated within this research subject to the following factors: • ratio of permeabilities of productive reservoirs (HP/ LP 10, 20, 30, 50) • ratio of thicknesses of productive reservoirs (HP/ LP1, 0.5, 0.4, 0.3) • water content of the product to start implementing the technology (0, 20, 60, 75, 85, 90 %) The factors may affect technology efficiency to a varying degree; therefore, an approach combining two other factors was used in this research for the suggested range of variation of water content figure 7-9.

Figure 7 .
Figure 7.Dependence of additional cumulative oil production on the ratio of thicknesses of productive reservoirs for options (Khp/Klp -ratio of permeabilities of productive reservoirs).
Figure 8.Dependence of additional cumulative oil production on the ratio of thicknesses of productive reservoirs for options (Khp/Klp -ratio of permeabilities of productive reservoirs).

Figure 9 .
Figure 9. Dependence of additional cumulative oil production on water content of the product, where implementation of the technology starts (thickness ratio is 0.3).

Figure 10 .
Figure 10.Distribution of permeability in a full-scale model.

Figure 11 .
Figure 11.Saturation distribution in a full-scale model.

Figure 12 .
Figure 12.Comparison of cumulative oil for various options.

Table 1 .
Comparing basic reservoir parameters, broken down by reservoirs