A new fracturing mode is put forward in the paper. Compared with the former mode, the new one has the following advancements:
Some key reservoir parameters such as effective permeability, effective thickness, loss coefficient, fracture azimuth and fracture height as well are all regarded as stochastic variables according to field statistic results;
Fracture length and conductivity are accordingly stochastic variables as a result of the random distribution of such fracturing treatment parameters as pumping rate, sand to liquid ratio, pad volume and sand volume as well;
As a result of undetermined reservoir and fracture variables, the post-fracturing rate is stochastic in most cases;
Monte Carlo method is utilized to perform economic analysis, and the relationship of net present value versus probability in a certain unconventional reservoir may be available on the base of above works, so that a decision-making policy may be determined accordingly.
After that, one field case is hydraulically fractured utilizing above strategy, and the post-fracturing effect is very satisfied, which indicates that the new overall hydraulic fracturing mode put forward in the paper is reliable and if it is put in to field applications, a greater economic benefits may be expected.
As we all know that hydraulic fracturing technology has been playing an important role in the economic development of low permeable reservoirs. However, there are still many low permeable and unconventional reserves in china, especially in Daqing, Changqing, Xinjiang and Jilin, etc., can not be developed effectively as a result of economic and technological considerations, so that they are still undeveloped up to now. What's more, the characteristics of above mentioned reservoirs are very complicated such as heterogeinity, anisotropy, less pay zone thickness distributed sparsely in vertical direction, discontinous formation deposits, and so on. In one words, as a result of stochastic distributions of reservoir parameters such as effective permeability, effective thickness, present formation pressure, leak off coefficient, etc., the fracturing treatment parameters such as pumping rate, PAD volume, proppant volume, sand to liquid ratio, etc., are all stochastic variables so that the fracture parameters such as fracture length, fracture height and fracture conductivity as well are all randomly distributed correspondingly. Therefore, the post fracturing rate and economic benefits are all undetermined, which indicates that there exists a great risk in developing above mentioned low permeable and unconventional reservoirs.
Consequently, it is very necessary to establish a risk analytical model of overall hydraulic fracturing in low permeable and unconventional reservoirs in china. The preliminary model put forward in the paper may be utilized to draw out a long term program in selecting objective reservoirs on the base of technical and economical analyses.
The Character of Low Permeable and Unconventional Reservoirs
very severe heterogeneous and anisotropic;
many parameters are stochastic distributed, such as permeability, thickness, leak off coefficient, etc..
very low permeability, normally less than 1×10–3μm2;
very low formation coefficient, normally ranged between 1.3×10–3μm2.m-7.3×10–3μm2.m;
low reserve quality, normally less than 42.2×104t/m2;
low pore pressure coefficient, normally less than 1.0;
thinner sandstone thickness distributed sparsely, with may shale barrier interbeded;
in most cases, there exists many fissures.