Abstract
Recent years have witnessed a renewed interest in development of coalbed methane (CBM) reservoirs. The success stories in San Juan basin and other CBM horizons have triggered this renewed interest. Many operators are asking questions regarding the most optimum way of producing CBM reservoirs. Drilling horizontal and multi-lateral wells are gaining popularity in many different coalbed reservoirs with varying results. This study concentrates on many variations of horizontal and multi-lateral wells and their potential benefits. It has been the rule of thumb that vertical wells are appropriate for thicker CBM reservoirs such as those found in San Juan basin and horizontal and multi-lateral wells should be used in thinner beds. Recently some operators are drilling horizontal and multi-laterals even in thicker beds and are claiming economic success.
In this study, we identify the most appropriate drilling patterns for coalbed methane reservoirs of different thickness and with different characteristics. The reservoir characteristics that have been studied include gas content, permeability, and desorption characteristics. The yard-stick for comparing different drilling configurations in this study is the net present value (NPV). This way cost of drilling has been taken into account when different horizontal and multi-lateral configurations are compared with one another. Furthermore, we have compared dual-, tri- and quad- laterals with fish-bone (also known as pinnate) configurations. In these configurations, the total length of horizontal wells as well as the spacing between laterals (SBL) has been studied.
Coal represents an unusual reservoir rock due to its highly complex reservoir characteristics. One of the characteristics that distinguish coal seams from conventional gas reservoirs is that coal represents both the source and the reservoir rock at the same time. Unlike conventional sandstone reservoirs, where the gas is found in free state within the pore structure of the rock, the methane gas is adsorbed onto the internal structure of the coal, which allows significant amount of gas to be stored in the coal rock[1].
Coal seam system is a naturally fractured heterogeneous reservoir characterized by macropores (fracture system, also known as cleat system) and micropores (coal matrix system). In general, the coal cleat system is orthogonal with one direction cross-cutting the other and varies from the case to the case, having significant impact on the coal deliverability1. The CBM production depends highly on the fracture system - fracture spacing and fracture interconnection.If the cleat system for any reason is not developed enough, the gas production could be very difficult. This occurs due to the low values of porosity and permeability in the matrix, making it almost inpossible for gas to move from matrix into the fractures. At the beginning, the coal system is in equilibrium and typically, water must be produced continuously from coal seams to reduce the reservoir pressure and release the gas. Gas from the coal can be produced only after initial dewatering of the system, and upon reaching low reservoir pressure. The dewatering process can take anywhere from few days to several months, which depends on CBM well configuration. Generally, the water production declines until the gas rate reaches the peak value. This ‘time-to-peak-gas’ is a critical parameter since the gas production starts declining after reaching the maximum[2]. Upon reaching the peak, gas production starts to decline and behavior of CBM production becomes similar to conventional reservoirs.
Methane can be drained through small diameter horizontal holes. One of the main advantages of the horizontal wells is that the well direction, shape, and position can be controlled3 . Using horizontal wells, an almost ideal position of the well can be determined and performed in respect to principal permeability directions of the coal. Proper positioning of the well as well as length of borehole drilled perpendicular to main fracture system of the coal, can significantly contribute to draining large areas.
Question is which shape of horizontal well to drill in order to produce larger amount of gas in the shortest possible time. Drilling horizontal wells in coal seams is highly important to the sweep efficiency. As the length and contact of well bore to the coal seam increase, the time for water production decreases. Usually, the gas production curve of vertical and horizontal wells will significantly differ from each other. In a very short time, the horizontal well will dewater the system, and significant water production will take place. The gas flow peak will occur soon after the well starts to produce. Probably, the most important part of the gas flow curve is the one after gas flow peaks. The slope of the curve is important, since the well will produce like conventional gas reservoir from that point. The flatter the curve, the better production will be for the rest of the well's life. The issue is that; horizontal wells are capable of draining the system very fast due to much longer well bore contact with the reservoir. Consequently, the adsorption process is triggered quickly and the well will elicit a very steep negative decline curve relatively fast. After reaching the peak, the gas production will start to decrease, again due to large contact area between horizontal well and the reservoir.
