The physical process and pressure-transient analysis considering fractures excessive extension in water injection wells

Abstract

It is well established within the industry that waterflooding almost takes place in the low permeability reservoir. In order to meet the multiple objectives of pressure maintenance, voidage replacement and sweep optimization, the waterflood-induced fractures can be reactivated when the formation pressure above fracture opening pressure. In general, this is a risky operation, which may lead undesired performance, i.e. pre-matured water breakthrough in producers, steep rise in oilfield overall water-cut profiles, unpredictable flood pattern, poor sweep and so on. However, neither the existed mathematical models nor current commercial softwares could simulate the bottom-hole pressure behaviors of the dynamic process of fractures propagation.The main objective of the study is to provide a dedicated methodology to analyze the pressure responses considering the dynamic process of the extension of waterflood-induced fractures. First, the physical mechanism of the mini-fractures initiation, communication, and propagation is presented. Next, we define the water injection flow coefficient, whose physical meaning is the characterization of the flow ability from fractures into matrix, with unit of m/d0.5. Several mathematical models are put forward to analyze the bottom-hole pressure behaviors considering the complex process of continuous evolution of main and secondary fractures. After that, parameter sensitivity, model comparison and verification are conducted. Finally, we apply the proposed models in a case derived from Changqing oilfield.Based on the proposed models, the pressure-transient behavoirs of wells with waterflood-induced fractures are obtained and type curves are plotted. The shape of these type curves is studied as a function of different relevant parameters, i.e. water injection volume, fracture area exponent, flow coefficient for main and secondary fractures, fracture surface area, secondary fracture initiation time and so on. It is concluded that the bottom-hole pressure is consisted of two parts: one is the increase of pressure caused by water injection while the other is pressure relief caused by fractures extension. Calculations indicate that the relationship between pressure logarithmic derivative and fractured injection time still meet the linearity approximately in log-log plot even considering fracture extension if constant fracture height is assumed. The slope of the curve is largely controlled by flow coefficient from fracture into matrix and fracture area exponent. Compared with the model of fixed fracture length (in the shut-in moment), the slope of pressure logarithmic derivative of this model proposed is smaller. The initiation of secondary fractures lowers the pressure derivative value. The results show pretty good agreements between our model and the fall-off analysis after shut-in period, with the relative error of 3.6%, which indicates our new models are reasonable.