Research On Sensitivity Of Tight Sandstone Reservoir For HH Oilfield

Tight sand reservoir of oil and gas resource is an area with huge potential. Tight reservoir are characterized by developed clay minerals, complex pore structures, low porosities and low permeabilities, usually less than 1×10-3μm2. Working fluids entering into the tight sand reservoir greatly decrease the productivity. Currently, there is no standard for tight reservoirs on the sensitivity damage evaluation. In our country, the techniques of sensitivity experimental evaluation for tight sandstone reservoir mainly rely on the industry standard as a reference. However, the existing standard SY/T5358-2010 Formation Damage Evaluation by Flow Test fails to account for start-up pressure with non-Darcy seepage of tight sand reservoir. Some of the shortcomings are:(1) Inadequate test coverage; (2) High requirements for laboratory equipment; (3) Neglect of stress sensitivity damage; (4) Long time for test stability. Consequently, there is the need to develop and improve the formation damage evaluation criteria for tight sandstone reservoirs.With the tight sandstone reservoir in HH Oilfield as a case study, the seepage characteristics of the reservoir were first analyzed. The matrix porosity in study area is mainly distributed between 6.27% and 12.64% with permeability ranging from 0.0100 ×10-3μm2 to 0.0772 X 10-3μm2. The pores and throats of matrix are tiny and the clay mineral types are rich in chlorite and kaolinite with a little amount of illite/smectite mixed layer. The average crack density of the micro-fracture developed in reservoirs can reach 0.88cracks/m. Therefore, the rock-forming mineral is subject to elastic and plastic deformation, resulting in a strong stress sensitivity of reservoir. The fluid flow in reservoir is very slow causing a nonlinear seepage flow with a starting pressure gradient.This study compared two sensitivity evaluation methods, constant-velocity and constant-pressure method and analyzed the relationship between pressure and liquid permeability change with time. The constant-pressure flow experiment method was recommended, based on its advantages stated as follows:(1) It usually takes continuous flow of 120mins to reach a steady state; (2) It overcomes the influence of stress sensitivity; (3) It overcomes the effect of start-up pressure gradient; (4) It overcomes the high requirements for experimental equipment and greater errors.Based on the study of reservoir seepage characteristic on HH Oilfield tight sandstone, this thesis established a set of formation damage evaluation methods for laboratory experiments of different types of HH Oilfield tight sandstone samples, including matrix sand sample and artificial fracture rock sample. These methods differ in the following aspects from conventional sandstone evaluation methods:(1) The selection of core sample and the preparation of core sample; (2) The test method of liquid permeability; (3) The time needed for the stability of permeability determination process; (4) The setting up mode of velocity for velocity-sensitivity evaluation method.Based on this set of formation damage evaluation methods for laboratory experiment, this study analyzed the damage mechanism for HH Oilfield tight sandstone reservoir. The results show that the matrix sample has weak velocity sensitivity, weak to medium weak water sensitivity, weak alkali sensitivity, weak acid sensitivity, and strong stress sensitivity. While the artificial fracture core sample has weak to medium weak velocity sensitivity, medium strong water sensitivity, weak alkali sensitivity, weak acid sensitivity, and strong stress sensitivity. Additionally, for the two sample types, the critical salinity is between 15658.75 mg/1 and 46976.25mg/1 and the critical stress is 5MPa.The results of HH Oilfield tight sandstone also show that the weak velocity sensitivity is attributed to small pore throat diameter and related clay particles(such as kaolinite) with larger diameter. The differences between water sensitivity and salinity sensitivity are related to the expansion of clay mineral and the quantity of migration clay mineral particles, particularly, chlorite and illite. Weak alkali sensitivity is associated with tiny content of related clay minerals (such as feldspar and mica) and small pore throat diameter. Acid sensitivity depends on the content of calcite and chlorite in the reservoir. Strong stress sensitivity is associated with tight reservoir and sheet-like pores as the main throat type.For sensitivity characteristics of tight sandstone reservoir in HH Oilfield, the following suggestions are proposed:controlling the drilling completion fluid density during production operation; adopting a drilling completion fluid system of high salinity; and taking preventive measures to cope with additional damage in the process of acidizing and fracturing.This research provides effective technical support for highly efficient and accurate evaluation of tight sandstone formation damage, as well as proposing the formation protection measures for the HH Oilfield formation damage. Finally, the study can also provide reliable basis for the stable and productive techniques such as solid-free drilling fluid.