Well-Production Challenges and Solutions in a Mature, Very-Low-Pressure Coalbed-Methane Reservoir

Abstract

Coalbed-methane (CBM) production in the San Juan basin of northwestern New Mexico and southwestern Colorado has spanned more than 30 years. Some parts of the field, such as the high-permeability Fairway, are now in a mature stage of reservoir-pressure depletion. Optimization of well-production operations in the Fairway presents many challenges because of its extremely low reservoir pressure (less than 100 psi in some areas), heavy coal-fines production, difficult artificial-lift challenges, increasing CO 2 %, and the presence of paraffin, inorganic scale, and corrosion.We use history matching by reservoir simulation to help diagnose the causes of well-production inefficiencies and then plan how to mitigate them. Simulation of Fairway wells typically require the use of an increasing reservoir-permeability trend caused by coal-matrix shrinkage with the desorption of methane and CO 2 . However, we have observed in some Fairway wells that below a reservoir pressure of approximately 300 psi, there is a flattening or even a decrease in the permeability trend. This shift in the permeability trend is likely caused by coal failure (i.e., a change in mechanical properties of coal) that is evidenced in the wells by an increased amount of coal-fines production.This paper is written in two parts. The first part presents the challenges we face in operating Fairway wells and the solutions we have developed to overcome them. Field observations and operating guidelines will be shared, along with well-intervention histories where we have seen success. The second part discusses our use of reservoir simulation to diagnose the causes of reduced well-production efficiency.

Experimental investigation on the impact of coal fines generation and migration on coal permeability

“…This results in significant drop in the gas productivity (Magill et al, 2010;Palmer et al, 2005). In addition, coal fines can migrate towards a CSG well, burying the downhole pumps, after which workovers will be required to resume the production process (Black, 2011;Okotie and Moore, 2010). Appropriate production strategies can alleviate the coal fines induced problems according to different reservoir conditions.…”

Integrated geomechanical, geohydraulic and geological (3G) study of coal fines generation and migration

“…Figure 1-1. Coal fines induced phenomena: (a) black effluent from coal seams (Okotie and Moore, 2010); (b) stuck standing valve ball from an in-seam pump (Okotie and Moore, 2010); and (c) unexpected gas production drops (Moore, 2012). ................................................................................ 5-7.…”

“…Some deposit at the bottom of the well, burying in-seam pumps ( Figure 1-1(b)). In this case, frequent workovers are required to clean out the fines to ensure gas can be produced properly (Marcinew and Hinkel, 1990;Nimerick et al, 1990;Okotie and Moore, 2010). Other coal fines may deposit inside coal cleats during the migration process, narrowing or even totally plugging these natural fractures.…”

“…Both phenomena detrimentally affect CSG production efficiency (Black, 2011). (Okotie and Moore, 2010); and (c) unexpected gas production drops (Moore, 2012).…”

“…The generated coal fines may plug fluid flow paths and reduce coal permeability, resulting in dramatic CSG productivity declines (Figure 1-1(c)). Coal fines may also accumulate at the bottom of a well, burying downhole pumps ( Figure 1-1(b)), which require regular workover (Okotie and Moore, 2010;Palmer et al, 2005). In this chapter, the concepts and models developed on formation fines damage in porous media are reviewed, and will be borrowed to establish the model framework on coal fines damage to the CSG reservoirs.…”