During the development of carbonate reservoirs in the Shunbei Oilfield, acid fluids were often used for fracturing in order to create a complex network of acid fractures connecting the fault-cave reservoirs. The acid fracturing network could be affected by many initial conditions, such as in-situ stress, faults distribution and internal initial fractures, and also by the construction techniques. In this study, the DFN-FEM method based on the ground stress simulation was used to study the three-dimensional distribution features of the fracture network produced by the acid fracture technology. Different factors, such as pumping displacement, acid viscosity and initial fracture distribution, were considered in the numerical investigations. The numerical results showed that: 1) the horizontal maximum principal stress was larger than the vertical stress with a main direction along the northeast in the study area; 2) cracks were relatively developed near the faults, and the direction of the fault strike is along the northwest; 3) during acid fracturing, the natural fractures perpendicular to the fault trend opened ahead of those parallel to the fault trend; 4) whenthe total liquid volume was kept constant and the viscosity of the liquid was changed, the resulting fractures developed with little changes; 5) when both the volume and the viscosity of the liquid were kept constant, the fractures propagated faster, and the extension communication range increased with the increased pumping displacement; 6) with the increase of the injected fluid volume, the extension range of the generated fractures became larger, and the generated fractures communicated with the natural fractures. The fracturing fluid leakage was mainly in the direction of the minimum principal stress.

1. Introduction

Shunbei Oilfield is a key exploration and development block of Northwest Oilfield Company in recent years. It was found that this reservoir had the such features as the ultra-deep (buried depth of 7300-8500m), ultra-high temperature (over 150°C), ultra-high pressure (maximum horizontal stress higher than 150MPa), strong heterogeneity, and the complexity distribution of fractures and caves etc. During the development process, most wells were drilled near the main strike-slip fault zones and got good production. Once the wells deviated from the faults, they needed acid fracturing to increase production.

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