Determination of the Hydration Damage Instability Period in a Shale Borehole Wall and Its Application to a Fuling Shale Gas Reservoir in China

Abstract

In reviewing Chinese shale gas reserves and national policies regarding shale gas exploitation, shale gas will be of critical importance in providing clean natural gas to China. However, compared to those in the United States, the cost of shale gas extraction and the complex problems encountered in more complex and deep drilling in China are key technologies that need to be overcome. Shale wellbore wall instability is a complex problem that often occurs during drilling. During the process of drilling in shale, the complex stress and fluid-structure interactions result in the wall rock generating a strong hydration diffusion and swelling effect, which alters the stress distribution in the rock wall and deteriorates the mechanical parameters of the rock. This results in instability damage of the shale wellbore wall. In this study, the stratigraphic stress characteristics of the Fuling Shale Gas Field were initially predicted, and the shale sample phase composition and development of bedding and microcracks were analyzed using X-ray diffraction and scanning electronic microscopy. The main driving potential difference function between the drilling fluid and shale was analyzed, and a radial adsorption diffusion model of the shale plane was established. Through a laboratory study, the space time change law of the water diffusion of the shale rock was assessed as well as the rock damage evolutionary law of the elastic modulus and compressive strength with water content. Then, combined with the shale hydration stress and strength deformation theory, a damage evolutionary equation for shale with water was derived, and the shale damage evolutionary limit equation and the method of determining the collapse cycle were established. Finally, the method was applied to the Fuling Shale Gas Field, the largest shale gas field in China, and a shale wellbore collapse cycle of approximately seven days in the field was obtained. The severity of economic loss resulting from wellbore wall instability was also determined. This study provides insight and guidance for reducing the costs of shale gas reservoir well drilling and efficient development.

Characteristics of Hydration Damage and Its Influence on Permeability of Lamellar Shale Oil Reservoirs in Ordos Basin

“…Compared with oil-based drilling fluid, water-based drilling fluid (WBM) has lower cost and better environmental protection, and is widely used in drilling engineering [11]. There is high clay mineral content and complex pore structure in shale oil reservoir, which leads to hydration reaction when drilling with water-based drilling fluid [12]. The hydration reaction will lead to the increase of porosity, the looseness of internal structure, and the decrease of rock strength.…”

Numerical Simulation on Borehole Instability Based on Disturbance State Concept

“…In the oil and gas industry, 50-80% of exploration and development costs are spent on drilling. The downhole accidents such as wellbore shrinkage, sticking, formation collapse, and leakage caused by borehole instability lead to an increase in the drilling cycle, the damage of downhole equipment, and an increase in the cost, which has restricted oil and gas exploration and development [1][2][3]. The core of high-quality, safe, efficient, and low-cost drilling is to evaluate the downhole surrounding rock environment, study the mechanism of borehole instability, and control the borehole instability, and it is of great theoretical and practical significance [4][5][6].…”

Application of a Novel and Sustainable Silicate Solution as an Alternative to Sodium Silicate for Clay Swelling Inhibition

“…During the drilling operations, around 75% of drilling problems and 90% of wellbore instability issues are associated with shale rock. 2 Therefore, the application of WBM without swelling inhibitors stipulates wellbore problems during the drilling operation. Oil-based mud (OBM) has superiority over WBM in terms of shale inhibition, lubricity, and stability at high temperatures.…”

“…Sensitive shale formations are more susceptible to wellbore instability problems due to hydration and swelling of clay in the wellbore formations. During the drilling operations, around 75% of drilling problems and 90% of wellbore instability issues are associated with shale rock . Therefore, the application of WBM without swelling inhibitors stipulates wellbore problems during the drilling operation.…”