It is difficult to determine the safe operation window of drilling fluid density (SOWDFD) for deep igneous rock strata. Although the formation three-pressure (pore pressure, collapse pressure, and fracture pressure) prediction method with credibility improves the accuracy of formation three-pressure prediction, it still has a large error for deep igneous strata. To solve this problem, a modified method of the SOWDFD in deep igneous rock strata is proposed based on the leakage statistics of adjacent wells. This method is based on the establishment of the SOWDFD with credibility. Through statistical analysis of drilling fluid density of igneous rock leaky formation group in adjacent wells, the fracture leakage law of the formation is revealed and the upper limit of leak-off pressure containing probability information is obtained. Finally, the modified SOWDFD with credibility for deep igneous rock strata is formed. In this work, the proposed method was used to compute the SOWDFD with credibility of SHB well in Xinjiang, China. Results show that the modified density window is consistent with the field drilling conditions and can reflect the narrow density window in the Permian and lower igneous strata. Combined with the formation three-pressure prediction method with credibility and the actual leakage law of adjacent wells, it can effectively improve the prediction accuracy of the SOWDFD for deep igneous rock strata. The findings of the study can help in better understanding of the complex downhole geological environment in deep igneous rock strata and making reasonable drilling design scheme.

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Horizontal well greatly propels the development of unconventional oil and gas. Considering the drilling safety and basic demand of hole cleaning, define length of horizontal well as extreme hydraulic extension length when the drilling fluid pump rate is equal to minimum cutting-carry pump rate and hole cleaning satisfied basic demand. Based on this concept, a prediction model is established according to the relationship among minimum cutting-carry pump rate, bottomhole pressure, circulation pressure loss and drilling pump pressure. This model considers the influence of cuttings on the hydrostatic column pressure, horizontal annular pressure drop and annular geometrical shape. And then this model is used to analyze the impact of well structure, formation property, drilling fluid property, hole cleaning degree and drilling parameters on extreme hydraulic extension length. On the above basis, a new index called ration of dispersion coefficient is introduced to evaluate the sensitivity of each factor. The sensitivity decreases as the sequence of cutting particle size, drilling fluid flow behavior index, acceptable cutting bed height, wellbore diameter, ROP(8-15 m/h), drilling fluid density, drilling string eccentricity, formation fracture pressure, drilling fluid consistency index, well vertical depth and ROP(1-8 m/h). Based on both mitigation effect and feasibility, cutting particle size, drilling fluid flow behavior index, acceptable cutting bed height and rate of penetration are worthy to optimize to prolong EHEL.