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

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 CO2%, 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 CO2. 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.