An Analytical Model for Pressure Transient Behavior of Acid-Fractured Well in Fracture-Cave Oil Reservoir: A Field Example from Tarim Basin in China

Abstract

AbstractIn fracture-cavity carbonate reservoirs, caves and fractures are the basic storage space. The SN oil and gas field in the Tarim Basin is one of this special fracture-cave oil reservoir. The main storage and seepage space are large caves and large fractures, and the pore space is large, which can reach above the meter level. It is very difficult to interpret this kind of oil reservoirs by the conventional triple medium well testing method. It is of practical significance to establish a well testing interpretation model for cave-type reservoirs and to interpret the well testing data of such reservoirs. Based on the typical well-fracture-cave configuration, we consider the finite conductivity of acid fracturing fractures. The fracture zone is regarded as a linear formation with finite conductivity and volume. Due to cavity characteristics of the cave, it is regarded as an equipotential media, and the internal pressure is equal in the all volume, and the cave supplies fluid to the outside in the form of storage flow. By coupling the acid-fracture flow, the flow in the fracture zone and the cave flow, two mathematical models for well test of acid fracturing in carbonate reservoirs are set up. Practical solutions of bottomhole pressure are obtained by using Laplace transform and Stehfest numerical inversion algorithm. The research results show that there are six or seven flow regimes exhibited: acid-fracture linear flow, with or without fracture zone storage flow, linear flow in fracture zone, cave storage flow, linear flow in fracture zone, cave storage flow, boundary flow. The distance between the acid-fracture and the cave affects the duration of the linear flow in the fracture zone. The greater the conductivity of the fractured zone, the more the linear flow of the fracture, the smaller the conductivity, the greater the seepage resistance, and the linear flow is converted into the reservoir flow of the fracture. Finally, a field example was explained by using the double logarithmic curve chart of the drilled cave model, and several parameters including acid-fracture, cave and the fractured zone were obtained by inversion. The production simulated based on the inversion results are in good agreement with the actual values. This paper presents two practical models to interpret the transient pressure responses of acid-fractured well in fracture-cave reservoir. The flow behavior is firstly investigated and the field example demonstrated practical use of the model to obtain the key parameters of acid-fracture, fracture zone and the cave.