An Evaluation Method of Shale Fracability Based on Stress Inversion

The evaluation of in-situ stresses and fracability is the basis of shale fracturing design. However, it is still challenging to establish a unified fracability evaluation method due to the unique structure and composition of shale. A new evaluation method of shale fracability based on stresses inversion was developed in this paper with a southeastern Sichuanshale block as the research object. The inversions of break down pressure and horizontal principal stresses were performed for reservoirs in the block with the fracturing data of 196 intervals of more than 10 wells after establishing a real-time conversion model for wellhead and bottom hole pressure. The relative error between inversion results and core test results was less than 10%. Depending on the comprehensive consideration of rock characteristics and fracturing parameters of the reservoirs, seven evaluation parameters were selected: the brittleness index, minimum horizontal principal stress gradient, fracture pressure gradient, difference coefficient of minimum and maximum horizontal stresses, comprehensive sand to liquid ratio, proportion of slick water, and proportion of 40/70-mesh and 30/50-mesh proppant. The weight of each evaluation parameter was determined by the coefficient-of-variation method. The resulting evaluation method for shale fracability was implemented to calculate the comprehensive fracability indexes of four wells subjected to blowout test wells in the target block, and comprehensive fracability index was found to be positively correlated well with test production. Specifically, comprehensive evaluation parameters were improved with respect to intervals of one well after the adjustment of fracturing parameters based on the evaluation and calculation of the comprehensive fracability index of fractured intervals with similar geological characteristics. The research shows that the new approach, which is an evaluation method of shale fracability based on stress inversion, can make full use of fracturing data to guide the fracturing of shale gas wells. It is of great significance for carrying out "one interval one scheme, fine fracturing" to increase the productivity of single well, and to enhance the overall development effect of the block.