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Wellbore instability is the primary technical problem that restricts the low-cost drilling of long-interval horizontal wells in the shale formation of the Jidong Oilfield. Based on the evaluation of the mineral composition, structure and physicochemical properties of shale, this paper investigates the mechanical behavior and instability characteristics of shale under fluid action by combining theoretical analysis, experimental evaluation and numerical simulation. Due to the existence of shale bedding and microcracks, the strength of shale deteriorates after soaking in drilling fluid. The conductivity of the weak surface of shale is much higher than that of the rock matrix. The penetration of drilling fluid into the formation along the weak surface directly reduces the strength of the structural surface of shale, which is prone to wellbore collapse. The collapse pressure of the shale formation in the Nanpu block of the Jidong oilfield was calculated. The well inclination angle, azimuth angle and drilling fluid soaking time were substituted in the deterioration model of rock mechanics parameters, and the safe drilling fluid density of the target layer was given. This work has important guiding significance for realizing wellbore stability and safe drilling of hard brittle shale in the Jidong Oilfield. [ABSTRACT FROM AUTHOR]

To provide a better alternative to the current shale inhibitors, this paper designs a novel aluminum-based shale inhibitor by complexing aluminum with humic acid (HA), which is expected to enhance wellbore stability in drilling. Firstly, the alkali dissolution and acid precipitation (ADAP) method was chosen to extract HA from lignite from Shanxi and Yunnan, China. Next, the complexation conditions for aluminum-humic acid (Al-HA) were optimized through orthogonal tests, including reaction ratio, temperature, time, and pH. The best complexation conditions for Al-HA-1 were determined as pH = 5.0, HA/NaAlO2 = 7/3, T = 75℃, time = 30 min, and those for Al-HA-2 as pH = 10.0, HA/ NaAlO2 = 3/7, T = 65℃, and time = 30 min. Further, several tests were conducted to evaluate the inhibition performance of our inhibitors. The results show that the hot rolling recovery of shale sample in 1.2 wt% Al-HA-2 solution was up to 90.52% after hot rolling for 16 h at 120℃; the linear swelling rate of artificial bentonite core in 1.2 wt% Al-HA-2 solution was merely 20.38% after 16 h. The inhibition mechanism of Al-HA mainly consists of flocculation on shale surface and chemical reverse osmosis. Both phenomena suppress filtrate intrusion and reduce pore pressure transmission. These superior properties make the proposed inhibitors a promising choice for wellbore stabilization, especially in shale oil/gas formations.

Wellbore instability is the primary technical problem that restricts the low-cost drilling of long-interval horizontal wells in the shale formation of the Jidong Oilfield. Based on the evaluation of the mineral composition, structure and physicochemical properties of shale, this paper investigates the mechanical behavior and instability characteristics of shale under fluid action by combining theoretical analysis, experimental evaluation and numerical simulation. Due to the existence of shale bedding and microcracks, the strength of shale deteriorates after soaking in drilling fluid. The conductivity of the weak surface of shale is much higher than that of the rock matrix. The penetration of drilling fluid into the formation along the weak surface directly reduces the strength of the structural surface of shale, which is prone to wellbore collapse. The collapse pressure of the shale formation in the Nanpu block of the Jidong oilfield was calculated. The well inclination angle, azimuth angle and drilling fluid soaking time were substituted in the deterioration model of rock mechanics parameters, and the safe drilling fluid density of the target layer was given. This work has important guiding significance for realizing wellbore stability and safe drilling of hard brittle shale in the Jidong Oilfield.

To provide a better alternative to the current shale inhibitors, this paper designs a novel aluminum-based shale inhibitor by complexing aluminum with humic acid (HA), which is expected to enhance wellbore stability in drilling. Firstly, the alkali dissolution and acid precipitation (ADAP) method was chosen to extract HA from lignite from Shanxi and Yunnan, China. Next, the complexation conditions for aluminum-humic acid (Al-HA) were optimized through orthogonal tests, including reaction ratio, temperature, time, and pH. The best complexation conditions for Al-HA-1 were determined as pH = 5.0, HA/NaAlO 2 = 7/3, T = 75℃, time = 30 min, and those for Al-HA-2 as pH = 10.0, HA/ NaAlO 2 = 3/7, T = 65℃, and time = 30 min. Further, several tests were conducted to evaluate the inhibition performance of our inhibitors. The results show that the hot rolling recovery of shale sample in 1.2 wt% Al-HA-2 solution was up to 90.52% after hot rolling for 16 h at 120℃; the linear swelling rate of artificial bentonite core in 1.2 wt% Al-HA-2 solution was merely 20.38% after 16 h. The inhibition mechanism of Al-HA mainly consists of flocculation on shale surface and chemical reverse osmosis. Both phenomena suppress filtrate intrusion and reduce pore pressure transmission. These superior properties make the proposed inhibitors a promising choice for wellbore stabilization, especially in shale oil/gas formations. [ABSTRACT FROM AUTHOR]

Supercritical carbon dioxide (ScCO 2) drilling can effectively protect shale formation from hydration damage and improve drilling rate comparing to conventional drilling technology. The wellbore stability of shale formation is one considerable issue under ScCO 2 drilling conditions. In this study, the numerical simulations are performed to calculate collapse cycling time of shale formation under ScCO 2 drilling conditions based on thermoporoelastic coupling model. The results show that comparing to water seepage condition, the variation of formation temperature is larger, pore pressure and stress are lower for ScCO 2 seepage condition without adsorption effect, the comparison between water and ScCO 2 seepage conditions verifies the thermoporoelastic coupling model. For ScCO 2 drilling conditions, if adsorption‒induced strain is ignored, the risk of wellbore collapse will be slightly underestimated comparing to the results with adsorption effect. When adsorption‒enhanced elastic modulus is ignored, the risk of wellbore collapse will be significantly underestimated comparing to the results with adsorption effect. The wellbore collapse may occur with the increasing well depth for ScCO 2 drilling conditions. This study can provide the theoretical guidance for exploiting shale reservoirs using ScCO 2. • The thermoporoelastic coupling analysis of wellbore stability for ScCO 2 drilling. • The effect of ScCO 2 adsorption-induced strain on wellbore stability. • The effect of ScCO 2 adsorption-enhanced elastic modulus on wellbore stability. • The comparison of distribution of temperature, pore pressure and stress between hydraulic fracturing and ScCO 2 fraturing. [ABSTRACT FROM AUTHOR]