A Comprehensive Approach to Increase Oil Production from Low Permeable Fractured Reservoirs with Multistage Hydraulic Fracturing
Introduction
The Ambitious development of hard-to-recover reserves from low permeability fractured reservoirs, such as the Bazhenov horizons, has been started relatively recently in the Russian Federation, and, to date, it is in a stage of experimental commercial development. The outcomes of the experimental commercial developments suggest that the estimated well flow rates were not achieved upon completion of wellflow back operations, due to which a more comprehensive approach was demanded, that would take into account the aspects of the geological and geomechanical seam structure and the process-dependent parameters of the formation stimulation. The Bazhenov Suite deposits have been developed in horizontal wells with the use of multi-stage hydraulic fracturing techniques. This being said, some complex reservoir structures of the Bazhenov Suite, as well as a high heterogeneity of its properties, both along the cross-section and within the whole area of the license blocks, make it impossible to create a universal approach for commercial oil production. This is also due to the fact that a small number of horizontal wells successfully operated with the use of multi-stage hydraulic fracturing could not provide a sufficient amount of statistical data about the outcomes of the performed multi-stage hydraulic fracturing operations. At present, one of the basic approaches to the development of such non-traditional targests, as the Bazhenov Suite, is to create an “artificial reservoir” or stimulated reservoir volume (SRV), which is an extensive network of fissures, making it possible to produce from a considerable part of a reservoir. Due to the significant differences of the Bazhenov Suite from a conventional reservoir (extremely low matrix permeability, natural fracturing, low anisotropy of rock stresses, etc.) it became necessary to make a hydraulic fracturing simulator which would enable the simulation of both planar fissures, and an extensive network. The Russia-made software system “ROST MGRP” (Calculation of the Optimal Fracturing System) has used as a simulator. The developed software product provides a way of simulating a multi-stage hydraulic fracturing with a glance to forming both stimulated reservoir volume, and single planar fissures. The given article presents a comprehensive approach to design, simulation and execution of hydraulic fracturing jobs in the conditions of a low permeable fractured reservoir, i.e. the Bazhenov Suite, taking into account both the geological and geomechanical properties of the target, and the multi-stage hydraulic fracturing job parameters. This approach resulted in an elaboration and the introduction of a plan for the execution of multi-stage hydraulic fracturing jobs, which made it possible to enhance the initial flow rate and cumulative production, compared with similar parameters of the previously drilled wells.
Development of the Geomechanical Model
The Bazhenov Suite reservoirs are featured with local disjunctive faults, natural fracturing, as well as heterogeneous strain-stress states. Consequently, to have a quality simulation of a multi-stage hydraulic fracturing job in a horizontal well, with a glance to forming an extensive network of fissures, it is first necessary to build a geological and geomechanical 3D model of a formation, taking an assessment of disjunctive faults, building a model of natural fracturing in the area of a projected well, as well as building a formation 1D geomechanical model for fissure initiation points i.e. multi-stage hydraulic fracturing stages. A 1D geomechanical model, built up for each stage of a multi-stage hydraulic fracturing job, is an upright projection of the target formation and its overlying strata, with key parameters determined, such as, Young’s modulus of elasticity, Poisson’s ratio, minimum horizontal stress, etc. The calculation of the inversion stress model, performed with the use of commercial software, resulted in a chart of thickness, dip angle, and strike of each type of fractures: shear fracture, extension fracture, and contraction fracture. Using the software complex “ROST MGRP” (Calculation of the Optimal Fracturing System), a 2D model of Discrete Fracture Network (DFN) was built for fractures of each type, in each calculation cell of the simulation area. The number of fractures in a cell has been rated according to the minimum and maximum values of their distribution density, obtained as a result of the calculated inversion stress model. The buildup of the DFN model of natural fracturing resulted in a map of fractures, with indication of each fracture predetermined position and dip azimuth.
The results obtained from the multi-stage hydraulic fracturing jobs performed in the Bazhenov Suite of drilled wells, and the results of the oil production simulation in accordance with the design of the MIPT Center for Engineering and Technology LLC makes it possible to conclude about the efficiency of the comprehensive approach to design, simulation, and implementation of hydraulic fracturing jobs for deposits of the Bazhenov Suite.
One more outcome of the 3D geological and geomechanical model of a formation, built with the use of the software package “ROST MGRP”( Calculation of the Optimal Fracturing System ) has been the stress field map of the simulated area, which introduces the values of the minimum and the maximum horizontal stress (anisotropy accounted for) and the value of the maximum stress azimuth (the azimuth of predominant hydraulic fracture propagation) in each calculation cell of the simulation area.
Development of Hydraulic Fracturing Design
Based in the outcomes of the post-event analysis, two basic hydraulic fracturing designs were singled out, which are now being used in the Bazhenov Suite wells, namely, fracs with hybrid-fluid system and crosslinked gel fracs. Some wells were drilled in the experimental commercial development area of the X-field, which penetrated pay intervals of the Bazhenov Suite.
Standard fracs were implemented in all the wells, their main aspects are presented in the Table on the previous page.
A seven-stage hydraulic fracturing job was carried out in the sidetracked Well 1, using crosslinked gel, whose consumption amounted to 5.5 m3/min. A crosslinked gel hydraulic fracturing job pumps an exclusive high viscosity gel with a constant increase in proppant concentration.
Crosslinked gel hydraulic fracturing job has the following set of advantages:
• it enables the use of a coarse fraction proppant
• high final proppant concentration;
• no proppant-settling problem in the bottom-hole area;
• hydraulic fracturing fissures are wide enough to transfer proppant.
Along with the advantages, some shortages are present, which are as follows:
Potential vertical growth of fractures (depending on mechanical property profile) which increases the likelihood that no-target intervals may be penetrated.
• The process is standard for thick permeable formations which are a direct opposite to the Bazhenov Suite deposits.
• After the proppant is delivered, the crosslinked gel is injected into the formation under high pressure, forms under the effect of a breaker fluid hard-to-
remove sediment, which can reduce hydraulic fracture conductivity.
The main limitation to the use of the crosslinked gels for hydraulic fracturing in the deposits of Bazhenov Suite is non-development of the stimulated reservoir volume (SRV). The formation of poorly branched fracture networks, or predominantly planar fractures, takes place, first of all, because of the high viscosity of the fluid, which often results in the scenario of intersections between the hydraulic fracture and an intrinsic fracture, without opening and consequently reinforcing the latter [5]. Such an approach can be applied for commingling of structural faults. Besides, using the standard crosslinked gel, operations suggests lower volumes of fluid, which causes lower half-length of the created hydraulic fractures, hence, the probability of intersection between natural fractures and hydraulic ones decrease. The hybrid design suggests the use of different types of fluid in the course of a single hydraulic fracturing operation. The use of large volume of low viscous fluids facilitates the expansion of man-made fractures and a more active formation of their extensive network (Fig.1). To reinforce the hydraulic fractures created with proppants, a large number of proppant slugs are used. However, when carrying out this type of hydraulic fracturing operation, a significant amount of fluid is injected, which results in the following complications:
• Formation of hydrodynamically isolated fracture network sections;
• Fractures are filled with low concentrations of proppant, which, while the well is operated, reduces the width and conductivity of a fracture to zero;
• Disintegration of the proppant, due to its low concentration, and being impressed into the walls of a fracture.
The analysis of the initial flow rates, including the trends of its decline, revealed that Well 4 and Well 1 have similar initial flow rates and cumulative production (Fig.2). Well 4 had a higher initial flow rate, but its rapid decline took place in the course of fluid withdrawal. One of the causes of this has been partial loss of SRV due to closing of unreinforced fractures, or the fractures having a low concentration of proppant, as it gets pressed into their walls. Well 1 had lower initial flow rate, however its current productive rate has been stable for almost 2 years now. The multi-stage hydraulic fracturing job performed in this well, used crosslinked gel, which resulted in formation of predominantly planar fractures. Well 2 and Well 3 are featured with higher initial and current flow rates, as well as with higher cumulative production. Lower fluid volumes were used in Well 3, with an unchanged average proppant weight, it has an extensive network of fractures which were created with a higher proppant concentration. Due to this, the conductivity of fractures in the course of well operation decreases less considerably. In the case of Well 2 the results are similar to the ones of Well 4. The similar injection design made it possible to achieve a maximum initial flow rate (circa 20 ton/day), however, it decreased considerably afterwards. Taking into account the gathered experience of performing hydraulic fracturing of the Bazhenov Suite reservoirs, the post hydraulic fracturing well operation analysis, the geological and geomechanical factors (heterogeneous stress fields, well area natural fracture map, 1D geological and geomechanical model for each stage), as well as the worldwide experience of developing shale deposits, the specialists of MIPT Center for Engineering and Technology “Bazhen” developed an alternative multi-stage hydraulic fracturing design for the conditions of Bazhenov Suite (Fig 3).
It corresponds to a hybrid hydraulic fracturing job comprising three different types of fluid: water with fraction reducers, linear gel and crosslinked gel. To avoid degradation of the fracture network, the total amount of fluid was reduced due to lower amount of overflush fluid used between proppant slugs. The use of large volume of low viscosity fluid facilitates more active formation of the fracture network. To more uniformly reinforce the formed fracture network, injection of proppant slugs with gradual increase in proppant concentration was proposed, which would contribute to reinforcement of massively smaller and distant fractures, in difference from the standard hydraulic fracturing design. Besides, the stepwise increase of proppant concentration contributes to increasing conductivity of fractures as the uninvaded stimulated reservoir zone nears the bottom hole area. To transfer proppant in low viscosity fluid, one of the major factors is using high rate hydraulic fracturing fleet. The high rate, in this case, serves to support the opening of the fracture system and compensation of leaking when the fracturing process becomes active. The use of relatively small amounts of crosslinked gel provides formation of cracks in a productive reservoir of Bazhenov Suite. The reduced amount of crosslinked gel provides the minimal contamination of fractures with products of hydrofrac gel decomposition. The simulation results (Fig.4) speak for an efficient use of the developed hydraulic fracturing design.
Hydrodynamic Modeling
To confirm the efficiency of multi-stage hydraulic fracturing operations based on various designs, as well as the predicted oil flow rate after multi-stage hydraulic fracturing operation, hydrodynamic modeling was carried out in the Perpendicular Bisection (PEBI) Hydrodynamic simulator. The given simulator makes it possible to calculate the predicted flow rate for fluid and oil, taking into account the formed fracture network, obtained in the simulator “ROST MGRP” (Calculation of the Optimal Fracturing System). Based on the results of the hydrodynamic modeling, the initial flow rate for horizontal wells with 15-stage hydraulic fracturing job amounts to 88 ton/day. The high simulated initial flow rate brings us to conclusion that the hydraulic fracturing design developed by the MIPT Center for Engineering and Technology has been effective. Due to this, it was decided to carry out a multi-stage hydraulic fracturing operation with alternative design in projected Well 5. Based on the results of the modeling, a 15-stage hydraulic fracturing job was planned in this well, with the use of alternative treatment design. The obtained SRV has greater number of cracks reinforced with proppant.
Conclusion (Data Analysis)
The hydraulic fracturing jobs carried out in Well 5, according to the approved work plan, had no complications. Fig.5 presents the dynamics of the predicted and actual oil production rate after bringing the Well 5 to stable production. The volume of the fluid injected in the course of the multi-stage hydraulic fracturing operation, as well as its further treatment, were taken into account in the material balance equations during the hydrodynamic simulation. After flow back of the well the initial oil flow rate of Well 5 amounted to 88 ton/day, which indicates a good convergence of it with the actual initial flow rate i.e. 83 ton/day. The actual trend of oil production decline agrees with the predicted one, which speaks for sufficiently good predictability of this model. The data obtained as the result of the multi-stage hydraulic fracturing jobs carried out in the wells drilled in Bazhenov Suite deposits, and the results of the oil production simulation according to the design developed by the MIPT Center for Engineering and Technology, make it possible for us to conclude that the comprehensive approach to design, modeling and carrying out of hydraulic fracturing operations in the formations of Bazhenov Suite are efficient. The hydraulic fracturing job carried out in accordance with the developed design resulted in considerable increase in stimulated reservoir volume (SRV), which has been confirmed both by the results of modeling based on actual injection volumes, and by the production rate increased by double, as a result of the treatment, compared with the flow rate of the neighboring wells stimulated in accordance with a standard plan.
Reference
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Authors
Rodionov V.V., Torba D.I., Kashapov D.V., Prodan A.S., Bochkarev A.V., Lisitsyn A.I. MIPT Center for Engineering and Technology LLC, RF, Saint-Petersburg D.F. Bukharov, O.V. Bukov Bazhen Technology Centre LLC, RF, Saint-Petersburg
