Abstract
Threshold pressure gradient (TPG) and stress sensitivity which cause the nonlinear flow in low permeability reservoirs were carried out by experiments. Firstly, the investigation of existing conditions of TPG for oil flow in irreducible water saturation low-permeability reservoirs was conducted and discussed, using the cores from a real offshore oilfield in China. The existence of TPG was proven. The relationship between TPG and absolute permeability was obtained by laboratory tests. TPG increases with decreasing absolute permeability. Then, stress sensitivity experiment was carried out through depressurizing experiment and step-up pressure experiment. Permeability modulus which characterizes stress sensitivity increases with decreasing absolute permeability. Consequently, a horizontal well pressure transient analysis mathematical model considering threshold pressure gradient and stress sensitivity was established on the basis of mass and momentum conservation equations. The finite element method (FEM) was presented to solve the model. Influencing factors, such as TPG, permeability modulus, skin factor, wellbore storage, horizontal length, horizontal position, and boundary effect on pressure and pressure derivative curves, were also discussed. Results analysis demonstrates that the pressure transient curves are different from Darcy’s model when considering the nonlinear flow characteristics. Both TPG and permeability modulus lead to more energy consumption and the reservoir pressure decreases more than Darcy’s model.
Highlights
With the growing tension of oil and gas resources, low permeability reservoirs have become the most important source of oil exploration and development in China’s petroleum industry
Many authors have noted the departures from Darcy’s law in porous media through experiment methods [1,2,3,4,5,6] and they found that the flow curve is a combination of a straight line and a concave curve
In 2004, Gavin [7] gave a detailed discussion on the non-Darcy phenomenon under low-velocity conditions in porous media
Summary
With the growing tension of oil and gas resources, low permeability reservoirs have become the most important source of oil exploration and development in China’s petroleum industry. Non-Darcy’s flow exists in low permeability porous media in which the TPG always can be observed. The non-Darcy flow has been discussed in many fields in petroleum including numerical simulation, enhanced oil recovery, productivity evaluation, and well test [11,12,13,14]. For low permeability reservoirs, this work has not been done by considering threshold pressure gradient and stress sensitivity which cause the nonlinear flow. Threshold pressure gradient and stress sensitivity experiments in low permeability oil reservoirs were made and discussed using the cores from a real offshore oilfield in China. The sensitivity of the type curves and derivative type curves on monitoring TPG, permeability modulus, well location, well length, wellbore storage, and skin factor was analyzed
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