Coupled Flow/Geomechanics Modeling of Interfracture Water Injection To Enhance Oil Recovery in Tight Reservoirs

Abstract

Summary Unconventional tight reservoirs that are typically characterized by low permeability and low porosity have contributed significantly to the global hydrocarbon production in recent years. Although hydraulic fracturing, along with horizontal well drilling, enables the economic development of such reservoirs, the production rate often declines sharply and results in low primary hydrocarbon recovery. The application of enhanced-oil-recovery (EOR) techniques in tight reservoirs has received much interest. In this study, the feasibility and efficiency of interfracture water injection to enhance oil recovery in multistage fractured tight oil reservoirs are analyzed through an efficient coupled flow/geomechanics model with an embedded discrete-fracture model (EDFM). A combined finite-volume/finite-element scheme is used to discretize the governing equations for flow and geomechanics, and the coupled problem is solved sequentially using a fixed-stress splitting algorithm. A basic numerical model consisting of a 15-stage fractured horizontal well is constructed using the petrophysical and geomechanical properties of a tight oil formation in Ordos Basin, China. Fractures indexed with even numbers are switched into injecting fractures when the production rate has dropped to less than a certain threshold. The improvement of oil recovery is analyzed by comparing the production profiles with and without water injection. In this coupled model, the fracture closure/opening during production/injection is considered according to the constitutive relations between fracture aperture and effective normal stress acting on the fracture faces. The poromechanical response of matrix is modeled by the Biot (1941) theory. The effects of fracture spacing, injection rate, and the presence of a natural-fracture network on oil-recovery enhancement are discussed through sensitivity analysis. The main mechanisms of interfracture water injection for enhancing oil recovery are waterflooding and reservoir-pressure maintenance. Small fracture spacing tends to reduce the oil recovery because of fracture interference and a limited drainage area; therefore, the primary depletion stage is shortened as the fracture spacing is reduced. The influence of interfracture water injection is more pronounced with smaller fracture spacing because the pressure-transient responses near the producing fractures are more dramatic considering the close proximity between the injecting fracture and the producing fracture. Although a higher injection rate results in higher oil recovery, the injectivity in low-permeability reservoirs limits the maximum-allowable injection rate. When secondary (natural)-fracture networks are considered, neighboring hydraulic fractures can be connected to one another via the secondary fractures, particularly if the interfracture spacing is small. Water can break through in the producing fractures quickly, which could also lead to high water cut and suboptimal oil-recovery performance. This study tests the feasibility and efficiency of interfracture injection to enhance tight oil recovery. The results indicate that interfracture injection can be a promising EOR technique for tight oil reservoirs, which sheds lights on future completion strategies and production design in tight reservoirs.

Experimental and Statistical Study on Wellbore Scaling Mechanisms and Characteristics for Huanjiang Oilfield

“…The Huanjiang oilfield is located in Huanxian County, Gansu Province. The work area starts from the west of Shancheng and ends at Qiaochuan in the east, and the area is about 3400 km [2]. The proven geological reserves are 335 million tons, and the geological reserves are 283 million tons.…”

“…Based on the experimental results from Table 1, the following conclusions can be drawn [1]: The formation water of deep Yanchang formation has a high content of scaling anions (HCO 3 -) and scaling cations (Ca 2+ ). During the uplifting process in oil depletion, the reduction of 6 Geofluids temperature and pressure leads to a large trend of inorganic scaling (CaCO 3 ) [2]. Each payzone contains a large amount of Na + , K + , and Cl -, which induces the crystallization of NaCl and KCl during the uplifting process and gradually forms the crystal scale on the wellbore [3].…”

“…Water injection is a mature and effective technology which has been successfully implemented worldwide. Although many oilfields have entered the tertiary oil recovery (EOR) stage, water injection to supplement formation energy and maintain formation pressure is still one of the basic measures for worldwide oilfields to ensure long-term stable production and improve oilfield development efficiency [1][2][3]. However, a series of production problems have been induced during the water injection process [4].…”

Applying two-grid method on unstructured tetrahedra to study fault stability and surface deformation in multiphase geomechanics

“…As shown in Fig. 2b, the two-grid approach is built on top of a staggered solution algorithm in which the flow and geomechanics subproblems are solved sequentially and iteratively using a fixed stress split iterative scheme [22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,21,46,47,48,49,50,51,52]. The accuracy of the two-grid method has been demonstrated for the classical Mandel's problem [53,54] in our earlier work [21].…”

Phase-Field Modeling of Pore-Scale Oil Replacement by Spontaneous Imbibition in Fractured Porous Media

“…2 Due to the nano-/microtron pore throat distribution and high fluid flow resistance in this type of reservoir, hydraulic fracturing is crucial to improve the relatively low production ability and achieve effective exploitation. 3,4 In fractured porous media, the microscopic oil droplets can be replaced with a fracturing fluid by imbibition and discharged from the fracture into the wellbore, thus increasing the production degree of original oil reserves. 4,5 The imbibition process mainly includes two classes: spontaneous imbibition and pressurized imbibition.…”