A Research On The Reinjection Capacity Of Sewage Reinjection Well In Gas Field

The composition of reinjection sewage in Yulin City and Mizhi County is quite complicated. Severe formation plugging in sewage reinjection process, which may lead to the increasing pressure in reinjection well and decreasing of injection rate, could stem from high suspended solid and oil content as well as excessive scale-forming ions in the sewage. Meanwhile, corrosion scale in the injection pipe work caused by low PH value and chloride ion and high-level and changeable pollutants as organic compounds and bacteria may bring about formation plugging. This problem may also result from out-of-standard alcohol treatment, inappropriate use of drug in drug-filling workshop and misconduct. Therefore, this research is urgent and of paramount importance in dealing with this problem.Based on the theories and methodologies of petroleum engineering, drilling engineering, oil and gas reservoir geology, water injection engineering, sewage disposal engineering, percolation theory and numerical simulation and analysis of oil and gas reservoir, this study summarizes the existing achievements and through a great deal of field investigations and various analyzing and evaluating methods, conducts analysis on the reinjection process, cut interval and movement of Well MZ-WS1, MZ-WS2, YL-WS1and YL-WS2. The study also conducts a thorough comparison of the converted water injection time, total water injection rate, water index, converted water injection rate and average water injection pressure among the four reinjection wells. Moreover, formation damage mechanism of reinjection well and causes of the increasing pressure in the reinjection well also constitute the research focus of this study. A pressure model is eventually set up and utilized, combined with the formulation of a reasonable working mode and policy for reinjection water.The results indicate that (1) in Yulin City and Mizhi County, high suspended solid and oil content in reinjection sewage, which cause formation pore plugging, can be intercepted by mechanical, adsorption and bridge interception methods. The excessive scale-forming ions in the sewage generate scale and inorganic matter plugging mismatched with formation water. Low PH value and chloride ion of reinjection sewage in Yulin City can produce erosions in the injection pipe work which may subsequently block the formation.(2) Core analysis in the adjacent area does not show speed sensitivity but indicates a sign of water sensitivity with the scale spanning from zero to weak. As a result, reinjection can be carried out under the condition of high reinjection rate based on the on-site situation. However, weak water sensitivity can lead to hydration swelling, causing the descending of permeability of formation. Nevertheless, measures should not be taken for this weak impact.(3) The sewage reinjection wells in this study are in (super) low permeable pay zones. The reinjection sewage contains great deal of suspended particles with large median, causing formation plugging. Research results indicate that sewage particles are mainly plugged in the water injection end on the core in the low permeable pay zones and can merely permeate into the center of the core. It is therefore concluded that sewage particles mainly plug the water injection end on the core and regular well cleanout and regurgitate well washing can release the blockage.(4) The study is generally appropriate in terms of technological process. But the treatment of sewage containing alcohol is beyond the standard, the use of drug in drug-filling workshop is inappropriate and the operation cannot meet the standard. We should focus more on these aspects and ensure that reinjection is carried out only upon reaching sewage reinjection standard.(5) It is inferred from the established math model that the faster the reinjection speed of sewage in single well is, the more rapid the increasing of pressure will be. At the same time, the reinjection rate will descend when the reinjection well is carried out to an extreme. Therefore, it is suggested that water injection speed should be reduced under possible conditions so that the water injection rate of the single well can be enhanced and the problem of accelerating pressure in water injection pump can be avoided.(6) The simulation results demonstrate that the reinjection capacity of the reinjection well in Yulin City far outweigh that of its counterparts in Mizhi County. In addition, the propagation speed of reinjection wells’ pressure also largely surpasses that of Mizhi reinjection wells. In terms of the scope of pressure, currently, the pressure has not reached the boundary of the sand body due to short reinjection time whereas the pressure of Yulin reinjection well has already reached the sand body boundaries.(7) It is concluded from the simulation results that the reasonable reinjection rate of reinjection well in Mizhi County should be controlled within100-150m3/d while the figure for reinjection well in Yulin City should be mastered under200m3/d. Considering the fact that the reinjection rate may exceeds this scope in the future (converted), it is recommended that the reinjection well should work continuously to maintain the reinjection rate within this scope.