Productivity analysis and numerical simulation for fractured wells in unconventional gas reservoirs

Abstract

Low-permeability gas reservoirs, which are widely distributed in China Offshore, could hardly be developed, due to the restrictions of the platform. LS gas field has the characteristic: prematurely senile appearance. That means it is currently in the middle period of development but has many features of the later period of the gas field’s life. There are two types of reservoirs in this gas field: conventional reservoirs distributed in the upper stratum and unconventional reservoirs with low porosity and low permeability distributed in the lower stratum. In order to compensate the decline of conventional gas production, the development of unconventional reservoirs has taken place during the past years. In LS gas field, the permeability of unconventional gas reservoirs ranges between 0.3 and 1, mD. The developing practice shows that the reservoirs with permeability lower than 1, mD don’t have natural productivity to maintain production. Hence, practices have been implemented gradually: acid fracturing, sand fracturing, low-cost fracturing based on platform, integration of completion fracturing, etc. After several years of practice, the sand fracturing technology was developed to be a proven technology and was selected preferentially. This paper presented: Design of sand fracturing, including the parameters such as sand amount, fracture size, etc. Equations (some are based on theories and some are based on experience) have been deduced to calculate productivity for production wells after sand fractured. Numerical simulation in accordance with the equations was applied in low-permeability gas reservoirs. Analyzing and evaluating the sand fracturing in different types of reservoirs. Calculation and prediction of the gradient stress could provide guidance for sand fracturing. Sensitivity and uncertainty analysis of the parameters in simulation. Furthermore, the future trend of unconventional reservoirs’ development was also discussed in this paper.