Challenges in Deep Shale Gas Drilling: A Case Study in Sichuan Basin

Abstract

As an unconventional gas resource, shale gas has been developed mostly in shallow reservoirs which are mainly buried in depth form 76 m to 2439 m in the world. However, the deep shale gas is drawing more attention as time goes by. In this paper, deep shale gas in Sichuan Basin is studied as an example where the shale gas reservoirs are mostly buried deep in depth from 2300 m to 4500 m. Because the shale gas in Sichuan Basin is buried deep with high formation pressure and the formation conditions are complex, there are many problems that will be encountered during drilling, such as wellbore instability, serious formation damage by high drilling fluid pressure, and low drilling efficiency, etc. In this paper, shale gas reservoir in Long maxi Group of Sichuan Basin is studied as a case for deep shale gas drilling. Basic characteristics of strata profile, lithological characteristics, formation fluids and formation pressure are analyzed combined with previous drilling data. Based on this, wellbore stability and formation damage are evaluated. The study indicates that in a relatively high borehole stress condition with a Mechanic-Chemical coupling, shale with lamellar, brittle and high clay content has a high collapse stress, and borehole trajectory design and drilling fluid performance are crucial to wellbore stabilization. Meanwhile, in deep shale drilling of this area, liquid phase will invade into bedding fissures and tectonic fractures to form a liquid phase trap between the fractures and matrix, so that the shale formation in near bore area will be isolated by the aqueous phase trapping and unable to penetrate, and it is the main formation damage. This will be helpful in improving wellbore stability in basin deep shale gas drilling, reducing formation damage, and promoting drilling and exploration efficiency.

Characteristics and Main Controlling Factors of Fractures within Highly-Evolved Marine Shale Reservoir in Strong Deformation Zone

“…The shale in Well YX1 is characterized by large thickness (nearly 40 m), high abundance of organic matter (average TOC of 5.8%), high-over maturity, presence of matrix pores (average porosity of 3.2%), and part of the fractures have a certain aperture, presenting favorable evidence for "geological sweet spots". Moreover, it is dominated by siliceous shale facies, with comparable elastic modulus, Poisson's ratio, and brittleness to other global shale gas reservoirs (Abousleiman et al, 2007;Sone and Zoback, 2010;Koesoemadinata et al, 2011;Kumar et al, 2012;Li et al, 2012;Wang R. Y. et al, 2016b), recording as good conditions for "engineering sweet spots". Therefore, the key factors for why the shale gas of Well YX1 cannot form industrial gas flow may be attributed to fault and fracture damage to the reservoir.…”

Wellbore instability in shale gas wells drilled by oil-based fluids

“…Water, white oil and kerosene can invade into shale to a certain degree [13] along with bedding planes when exposed to the shale. White oil and kerosene are the main components of oil based drilling fluids used in shale gas reservoirs.…”

Estimating Drillpipe Body Wear to Improve Drilling Performance in Challenging Shale Plays

“…Through several field case studies, they also demonstrated the importance of stick-slip considerations, some mitigation techniques, and issues related to several downhole heating failures resulting from excessive wellbore friction in the horizontal sections. Li et al (2012) described the challenges in deep shale gas drilling through a case study in the Sichuan basin where reservoirs are primarily buried deep, from approximately 7, 500 ft to 15, 000 ft. They discussed the issues related to wellbore instability, serious formation damage resulting from high drilling fluid pressure, and low drilling efficiency that is encountered while drilling in a high formation pressure basin and under complex formation conditions.…”

“…White et al (1987) presented a casing wear estimation technique using laboratory measurements and performed field predictions; additional casing wear modeling methods are presented in literature by Kumar et al (2013) and Samuel et al (2014). While addressing challenges related to horizontal drilling applications, Samuel et al (2013) also documented all existing casing wear models together that provide an insight into the current problem.…”