Study On Productivity Of Fractured Horizontal Wells In Low Permeability Oil Reservoirs

With the rapid development of economy, people’s demand of oil and gas resources is increasing day by day. After a long term development, the remaining development potential of old oilfield is insufficient, the production of old oilfield has been unable to meet the needs of people. Low-permeability oil and gas resources are widely distributed and have huge reserves in China. Therefore, people gradually turned to low-permeability oil and gas resources, and treat them as a complement to conventional energy source. The development of low-permeability reservoirs is very difficult. At present, horizontal well combined with hydraulic fracturing technology is the main technology in developing low-permeability reservoirs, but the cost of this technology is very high. As well productivity is directly related to the economic benefits of reservoir development, the productivity of fractured horizontal well is the focus of people’s attention. At present, the calculation of productivity of fractured horizontal well in low-permeability reservoirs is immature. The research on the optimization of horizontal well with multiple hydraulic fractures is few. So the study of this paper is of great theoretical and practical significance.This paper selected Chang 8 reservoir of Baibao oilfield in Ordos Basin as the research object. By combining experiments, theoretical analysis and field practice, it has studied the flow characteristics, productivity prediction and fracturing parameter optimization.(1) The analysis of flow characteristics through laboratory experiments. Based on core experiments (permeability measurement, sensitivity experiments and single phase flow experiments), it is found that the average porosity is 10.9%, the average permeability is 0.87 mD, the average threshold pressure gradient is 0.034 MPa/m. All the results suggest that Chang 8 reservoir belongs to ultra-low permeability reservoir, it is difficult for oil to move in the reservoir.(2) Study of the flow behavior of fractured horizontal well. According to the size and distribution of ground stress, it can form transverse fracture, longitudinal fracture or horizontal fracture after fracturing. Transverse fractures’have the best stimulation effect. This paper selected transverse fractures as the research object. Oil flow in Z plane was transformed to linear flow in W plane with conformal mapping. And the flow in fractures is radial flow, the flow radius is half length of fracture.(3) The analysis of Ning Zhengfu equation. Ning Zhengfu equation is a equation for calculating the productivity of fractured horizontal wells. In this equation, a coordinate system was established, in which the central of horizontal well was the zero point. This equation is suit for the situation of equally spaced fractures, but in actual situation the spacing of fractures may not be equal. As the half-length of fractures are usually greater than the formation thickness, it is not appropriate to treat the flow in fracture as a radial flow. Due to the gathering of oil in wellbore, there is a pressure drop in the wellbore, the pressure at the bottom of fractures are not equal to the bottom hole flowing pressure. Therefore, this equation needs further improvement.(4) Model establishment and solution for fractured horizontal wells’productivity prediction. A productivity prediction model was build based on Ning Zhengfu equation. In this equation, a coordinate system was established and the zero point was the heel of horizontal well, so it is suit for the situation of unequally spaced fractures. Threshold pressure gradient and wellbore pressure drop are contained in this equation. The flow in fractures is radial flow. The production of each fracture was obtained by iterative method, then the well production can be get by adding the production of each fracture.(5) Analysis of factors affecting the productivity of fractured horizontal wells. Different productivity can be calculated by inputting different formation parameters and fracture parameters to the calculate program. It is found that productivity increased with permeability, but the increasing rate decreased gradually. Productivity decreased with the viscosity of oil, but the decreasing rate decreased gradually. Productivity increased with formation thickness, but the increasing rate decreased gradually. Threshold pressure gradient has great influence on productivity, productivity decreased with threshold pressure gradient. Productivity increased with the number of fractures, but the increasing rate decreased gradually. Because the distance between fractures decreased with the increasing of fracture number, and this increased the interaction of fractures. The half-length of fractures has great impact on productivity, productivity increased with the half-length of fractures. Productivity increased with flow conductivity of fractures, but the increasing rate decreased gradually.(6) Optimization of fracture parameters. For given formation parameters and a given length of horizontal wellbore, calculate the productivity by assuming different fracture parameters, then choose the best fracture parameters.