Optimal Length Design Of Horizontal Well In Low Permeability Heavy Oil Reservoir

This paper deduces the productivity formula of horizontal wells. Short horizontal wells are those which lengths are much shorter than the size of drainage area. The conventional production rate equations are all established for short horizontal wells, which can not be used to calculate the production rates of long horizontal wells. So called long horizontal wells are those which lengths are almost the same with the size of drainage area. Based on the equivalent flow resistance theory, a production rate equation was derived in this paper, which can be used for production rate calculation of long horizontal wells, but can not be used for that of short horizontal wells, otherwise, the production rate may be overestimated. The production rate calculated by the long horizontal well equation is the lower limit of production rate calculated by horizontal well equations.Because of friction loss in horizontal well-bore, pressure drop of horizontal well-bore may equal to that of reservoir pressure drop, which results that the reservoir pressure drop at the tip of horizontal well-bore is rather small or equals to zero, this phenomenon often appears in high permeability or coning reservoirs, pressure difference of these reservoirs is low or limited. Therefore, studying the design method of optimal horizontal well-bore length has a guide meaning to development program design of horizontal wells.By dividing three reservoir type cases( bottom water drive reservoir, gas cap and bottom water drive reservoir, gas cap drive reservoir), the paper derives flow model in reservoir, flow model in horizontal well-bore, the optimal length model of horizontal well-bore. In the process of derivation, the factors, such as flow regime( laminar flow, turbulent flow) and relative roughness of well-bore, which have influence on productivity of horizontal wells, are considered.Finally, there are three optimization principles for horizontal well-bore length. Friction loss: the optimal horizontal well-bore length is the length that the production decreases by20% due to the friction loss. Ultimate production: a formula is derived from the ma-thematic model of optimal horizontal well-bore length to calculate the ultimate output for horizontal well in definite reservoir and fluid condition. Techno-economic appraisal: horizontal well optimum length double-criteria problem is set and solved.