Recent Advances in Coal Gas-Well Openhole Well Completion Technology
Authors: Mavor M.J. 
Published: December 2007

Distinguished Author Series

This paper reviews the current applications of openhole techniques in the San Juan basin. Examples of recent advances and details for the use of the technology are documented in many of the references. The range of reservoir properties required for successful implementation of the technology is included. Hypotheses proposed to explain the success of the openhole wells are presented. The paper concludes with a discussion on future advances required to improve current completion procedures and the potential applicability of the process to other reservoir rock types.

Introduction

Production of natural gas from coal seams has reached commercial levels during the past 2 decades in the San Juan basin of Colorado and New Mexico and the Warrior basin of Alabama. Because of the susceptibility of the naturally fractured coal to formation damage, the techniques used to drill and complete wells have a dramatic influence on fluid productivity. The greatest gas and water production rates have been achieved from the Fruitland Formation of the San Juan basin in wells completed with dynamic openhole techniques commonly referred to as openhole cavity completions. The dynamic completion procedure consists of gas and water injection into the reservoir followed immediately by a controlled blowout of the well that is repeated over 1 to 2 weeks. In many cases, the dynamic openhole wells outperform adjacent wells completed with cased-hole hydraulic fracturing by an order of magnitude. Typically, the relative performance ratio between the two completion types is three-fold. Formation damage surrounding the cased-well induced fractures is the primary cause of the productivity differences between the completion types.

Dynamic Openhole Completion Procedures

The state-of-the-art coal gas-well openhole completion technique allows coal and other rock to collapse into the wellbore during a controlled blowout, resulting in an enlarged wellbore in the coal zones. "Openhole cavitycompletion" is often used because measured diameters of enlarged wellbore shave ranged from that of the bit diameter to 4.8 m [16 ft ]. "Dynamicopenhole completion" more accurately describes the technique because creation of a cavity is a byproduct of the process, not the primary objective. The objective of a dynamic openhole completion is to link the wellbore with the undamaged natural fracture system of the reservoir. During the process, damaged, near-wellbore coal and other rocks are removed; multidirectional, self-propped fractures are created that intersect pre-existing natural fractures; the near-wellbore aperture of pre-existing natural fractures may be increased and retained; and the enlarged wellbore may intersect natural fractures. The wells produce gas and water at rates controlled by the normal components of Darcy's law (e.g., the pressure differential into the enlarged wellbore and the absolute and relative permeabilities of the reservoir).

An excellent example of the improved gas productivity of openhole wells compared with that of cased, fractured wells can be taken from the Northeast Blanco Unit (NEBU) in the San Juan basin (see Fig. 1). The NEBU No. 403 Ropenhole well has produced at rates exceeding 200 000 std m3/d and 91 m3/d[7,000 Mscf/D and 570 STB/D] of gas and water, respectively, at a bottomhole pressure (BHP) of 6400 kPa [930 psia]. The NEBU No. 403, some 61 m [200 ft] from NEBU No. 403 R, was completed in the same coal gas reservoirs with cased-hole, crosslinked gel fracture stimulation techniques. The maximum productivity of the cased well was 31 000 std m3/d and 64 m3/d [1,100 Mscf/Dand 400 STB/D] of gas and water, respectively, at a BHP of 500 kPa [800 psia]. Table 1 lists the locations of these and other wells discussed.

Numerous other examples can be selected to illustrate the success of the openhole completions within a region of the San Juan basin referred to as the openhole "fairway." According to a recent study, the average fairway openhole well recovers 5.9×107 std m3 [2.1 Bscf] of gas in 3 years compared to1.4×107 std m3 [0.5 Bscf] of gas from cased, fractured wells during the sametime. More than 600 openhole wells are on production, accounting for 73% of the coal gas production from the basin. More than 920 openhole wells have been drilled, accounting for 33% of the San Juan coal gas wells.

Fig. 2 compares average gas production rates from openhole and cased, fractured wells located in and outside the fairway over the first 3 years of the well life. The average gas production rate of fairway openhole wells is more than five times greater than that of the cased, fractured wells after 3years. In addition, the openhole well productivity has not reached the maximum productivity that is expected as water is depleted from the natural fracture system. The relatively constant gas production rate of the fractured wells is commonly associated with formation damage. In contrast, outside the fairway, the cased, fractured wells outperform openhole wells, but the productivity of both types is less than that within the fairway. The productivity differences between inside and outside the fairway are believed to result from the difference in the absolute permeability of the natural fracture system, although other properties, such as gas content, also differ. In the greater-permeability reservoirs, the openhole wells outperform the cased, fractured wells. Cased, fractured wells located in lower-permeability reservoirs are less susceptible to damage. Therefore, casing and fracturing wells can be the preferred completion technique.

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