Study On Optimization Approach Of Well Completion For Horizontal Well In Low Permeability Reservoir

Low permeability reservoir takes up large proportion in oil&gas reservoir of China and horizontal well is an important technique to develop it economically and efficiently. Completion not only influences the successful well operation and well life but also directly affects the production ratio of low permeability gas reservoir. Starting with the geological characters of low permeability gas reservoir and taking the specific requirements of drilling engineering and production engineering into account, this essay studied the completion optimization of low permeability gas reservoir, which has important significance in the improvement of production rate per well, lengthen of well life and the incensement of economic benefits. Based on the fundamental theories and methods of rock mechanics, the influence of in-situ stress environment, enhanced oil recovery operation, formation pressure failure and exploitation method to the critical bottom hole pressure and appropriate completion of open hole horizontal well is analyzed by theoretical calculation. Furthermore, by synthesizing various factors including reservoir geology, geo-mechanics and engineering requirements, completion optimization study and software development have been done using fuzzy mathematics theory. Combining on site gas reservoir data and completion optimizing method, the practical engineering application is also presented. Main results and achievements as follows:(1) In-situ stress states is one of the most important factors influent the well completion. In the same formation, critical bottom hold pressure and initial pressure of horizontal wells with different trajectories change with stress states. The completion could be very complex and effect of stimulation technologies could be ineffective due to inappropriate well trajectory. Therefore, drilling and completion should consider integratively and the completion optimization should start from trajectory optimization. Under the three types of in-situ stress conditions used in this thesis:①Under the same rock strength, stress state and the orientation of the borehole is changed, the critical bottom hole flowing pressure from the wells of the strata fracturing pressure will change. When the horizontal well in bottom of the well fluid column pressure below the critical bottom hole flowing pressure, the borehole wall will collapse,When the horizontal well critical bottom hole flowing pressure is higher than the formation pore pressure, not open hole completion of horizontal wells To get the capacity, we must support the sidewall.②Under the potential normal-fault stress states, horizontal well paralleled with maximum horizontal stress has relatively weak stability but likely to form fracture plane along with horizontal well. Horizontal well paralleled with minimum horizontal stress has relatively strong stability but initial pressure is much higher and the fracture plane intersects with plane of vertical stress and maximum horizontal stress.③Under the potential reverse-fault stress states, horizontal well paralleled with maximum horizontal stress can keep stable in lower bottom well pressure fracture pressure is relatively high. Horizontal well paralleled with minimum horizontal stress shows contrary features. Under the stress states, horizontal fractures are formed parallel with well trajectory.④Under the potential strike slip stress states, horizontal well paralleled with maximum or minimum horizontal stress shows similar features in self-stability of formation around horizontal wellbore and difficulty hydro fracture with wells under reverse fault stress states. Vertical fractures paralleled with well trajectory and intersect with minimum horizontal stress is likely to formed in horizontal well paralleled with maximum or minimum horizontal stress and vertical fractures perpendicular to well trajectory is easily to formed in horizontal well paralleled with minimum horizontal stress.(2) The impact of depletion and acidizing of low permeability gas reservoir on the completion optimization shouldn’t be ignored. Different type of acidizing fluid system, acid functional time and effect degree have different effects on the completion. Acidizing and depletion can’t guarantee the normal and long-term safety production of gas well. Therefore, the necessity of stimulation is not the only factor need to be considered in low permeability gas reservoir completion but also the impact of stimulation on formation self-stability should also be taken into account. As in this thesis research conditions, acidizing、depletion and the combined action reduce the critical drawdown pressure significantly in open hole horizontal well. In some cases, completion may change from unsupported to support way.(3) Based on the comprehensive analysis of completion influence factors, considering the feature that completion optimization involves many uncertain factors including engineering、 geology、economic benefits and production ration, an completion optimization method based on fuzzy comprehensive evaluation theory is established and a corresponding software is developed.A practical application is also presented in this thesis.