Flow-Testing Coalbed Methane Production Wells in the Presence of Water and Gas

Abstract

Abstract When water pressure is lowered to allow coalbed methane to desorb and flow to the well, three distinct flow regimes can be identified:Saturated water flow with no gas phase,Unsaturated water flow with an immobile gasphase, andTwo-phase flow of gas and water. This paper derives a solution which includes the first two regimes in a single formula. The concept of parameter measurement windows is introduced and applied to analyze data from Glover no. 1 well drawdown test no. 2 to obtain additional information on relative permeability and saturation curves. An increase in slope, which was initially attributed to other causes, is explained as a relative permeability effect. Introduction Gas production from coalbed methane reservoirs may follow three stages as the reservoir pressure declines from reduction of hydrostatic pressure by pumping off Ne water in the reservoir (Fig. 1). pumping off Ne water in the reservoir (Fig. 1). Most coalbed methane reservoirs are found to be under near-hydrostatic pressure and are saturated* with water. Methane is held within the porous coal matrix by an adsorption mechanism which is controlled by the reservoir pressure. When a water-saturated coalbed methane well is first produced, it is common to encounter only single-phase or saturated flow, i.e., only water is produced. This is stage 1 where only one phase exists and pore spaces are fully water-saturated. As water is removed and reservoir pressure is reduced further, methane gas bubbles begin to form as the result of desorption from the coal, and pore spaces are partially saturated with water. The bubbles block partially saturated with water. The bubbles block some of the pathways which were originally available to water flow; thus the relative permeability of the formation to water is reduced. However, the gas does not yet flow because the bubbles are not connected within the porous coal matrix nor in the cleat or natural fracture system of the coalbed. This second stage is called an unsaturated, single-phase flow regime where, although two phases are present (water and gas), only the water phase is present (water and gas), only the water phase is mobile. Stage III is reached as the reservoir pressure decreases and additional gas is desorbed. pressure decreases and additional gas is desorbed. The gas saturation builds until the gas bubbles connect and form a continuous pathway to the wellbore. As shown on Figure 1, two-phase flow begins at the point where the relative permeability to gas becomes non-zero. (Note that the slopes of the curves on Figure I are exaggerated for illustrative purposes.) As the reservoir pressure is further purposes.) As the reservoir pressure is further reduced and water saturation declines, the relative permeability to gas increases at the expense of the permeability to gas increases at the expense of the relative permeability to water. This sequence of regimes progresses outward from the wellbore into the formation over time, i.e., when two-phase flow occurs at the wellbore, unsaturated and saturated single-phase flow occur simultaneously further into the formation. RESERVOIR CHARACTERIZATION PARAMETERS Table 1 lists the reservoir characterization parameters which may be important in predicting parameters which may be important in predicting long-term methane production from coalbeds and in designing wellfields. Of the 11 parameters listed, only three (reservoir pressure, porosity, and compressibility) may be measured with some confidence without well tests. If the permeability is very low (on the order of microdarcies or less), then even reservoir pressure must be estimated using recovery well tests.