Challenging Foam Sand Cleanout Operation in Ultra-Low Reservoir Pressure Wells Resumes Production in a CBM Field

Abstract

Raniganj coal bed methane (CBM) field in India has over three hundred wells producing gas since 2010. Over time due to extensive dewatering, reservoir pressures have dropped to sub-hydrostatic levels, with some wells having water levels as low as 500m below the surface (~0.3 psi/ft gradient). Due to continuous production, the sand influx is ubiquitous in these moderately deviated wells requiring efficient well cleanout methodologies to continue production from old wells which have been shut due to sand ingress into the wellbore. This paper illustrates a detailed case study of effective well cleanout using Foam Assisted Coiled Tubing Cleanout. This methodology helped established standard practices across the entire field for foam cleanouts in old shut-in wells, helping in resuming gas production and avoiding expensive infill well drilling. Conventional two-phase nitrified cleanouts are sub-optimal due to very low bottom hole pressure (BHP) which requires higher nitrogen (N2) pumping rates and still had insufficient annular velocities considering 1.75-in. diameter coiled tubing (CT). Also, this poses a greater risk of the CT becoming stuck in the event of sudden lost returns. Hence, to establish circulation, all the existing thief zones need to be plugged. Using conventional lost circulation materials like calcium carbonates was not feasible due to the uncertain and expensive post-job clean-up required to restore zone permeability. Hence foam was selected for temporary zone plugging which can restore zone permeability automatically once foam disintegrates as per its half-life. Various foam recipes were tested to achieve the desired foam life, as it was imperative that it exceeded job treatment time. Also, localized work instructions were developed as per coal seam behavior, initial frac treatment pressures, and the production history of the well. These work instructions were then later developed into a standard operating practice (SOP) after achieving successful well cleanouts in a few initial pilot wells. In these 5.5-in. monobore completion wells, effective zonal isolation was required to sustain the higher flow velocities required for cleanouts. This was ensured by several CT simulation iterations aiming for foam quality of at least 75%. In absence of downhole pressure data, BHP was calculated from frac bottom hole treating pressures and ISIP. Also, the volumes of the foaming agent were optimized, and benchmarks were set for when to stop pumping the foaming agent and start taking pre-calculated sand bites. Considering that significant amounts of sand were to be cleaned out, it was important to calculate bite size, simulate the solids transport and maximum additional pressure head created during cleanouts. This paper presents a detailed case study of conducting Foam Cleanouts using Coiled Tubing in CBM wells having ultra-low BOTTOM HOLEP and big completion size of 5.5-in. The paper also describes in-depth procedures, lessons learned, and CT simulations involved which resulted in safe, successful, and efficient well cleanout jobs in over 50 wells.