Optimization method of horizontal well orientation and layer selection in low-rank thick coal seam: A case study of Jiergalangtu area

In order to solve the optimization problem of horizontal well orientation and layer selection in low-rank thick coal seam, based on low-rank coal stress sensitivity mechanism and horizontal well gas production model with multiple oblique fractures, the influence of horizontal stress difference and fracture on horizontal well orientation are studied quantitatively.Through weak plane theory and fracture mechanics, the wellbore stability calculation model of coalbed methane (CBM) well is established, the comprehensive selection method of geologic logging and fragility index for thick coal seam is put forward.The research shows that:① the coal compression coefficient in southern Qinshui basin is 0.038-0.077, the coal compression coefficient in eastern Ordos basin is 0.025-0.074, the compression coefficient of low-rank coal is greater than that of high-rank coal, the compression coefficient of wet coal sample is greater than that of dry coal sample; when pressure rises, the compressibility coefficient is larger than that of depressurization, the decreasing range of crack sample is greater than that of natural sample.② The horizontal stress difference would lead to permeability anisotropy.The compression coefficient and horizontal stress difference are the two most important impact factors of horizontal well orientation, when horizontal stress difference is greater than 8 MPa in low-rank coal seam or 14 MPa in high-rank coal seam, the optimal orientation is approximately parallel to the direction of minimum horizontal stress.Gas production is optimal when the angle between wellbore and fracture is greater than 60°, the sensitivity of gas production on fracture width decreases significantly when fracture width is greater than 3 mm.③ When face cleat dip angle is less than 25°, drilling along dip direction and strike direction are the maximum and minimum safe drilling fluid density window, respectively.When face cleat dip angle is greater than 25°, drilling along dip direction is still the maximum safe drilling fluid density window, but the minimum safe drilling fluid density window is between dip direction and strike direction.④ The optimal horizontal well orientation in Jiergalangtu area is NE90°-114°, the gas production of well JM-X indicates the method could guide the layer selection of thick coal seams effectively.