FROZEN HEAT | Volume 2

Box 3.2 Studies assessing the depressurization production technique

The first quantitative studies of the response of a gas hydrate reservoir to pressure drawdown were carried out as part of the 2002 Mallik research and development program in Canada’s Mackenzie Delta. Using Schlumberger’s Modular Formation Dynamics Tester (MDT) wireline tool, small-scale pressure drawdown tests within 0.5-metre-thick perforated intervals were undertaken in a variety of reservoir settings (Dallimore and Collett 2005). Complementary core data and well logs supported the detailed assessment of formation porosity and permeability and gas hydrate saturation. These studies confirmed that the gas hydrates occurred within the pore space of fine- to coarse-grained sands with low but measurable permeability. The pressure response and observed gas and water flows confirmed that, in spite of the low reservoir permeability, it was possible to transmit a pressure drop into the formation and induce in situ gas hydrate dissociation. Fine- and coarse-scale heterogeneity was also documented, as well as the occurrence of natural fractures within the gas hydrate reservoir. The MDT tool was also used in 2007 as part of a drilling program on the Alaska North Slope (Hunter et al. 2011). In this case, MDT testing was undertaken in an open-hole condition, rather than in the cased hole condition at Mallik, providing evenmore reliable measurements. The interpretations indicate measurable permeability in four discrete zones with differing reservoir properties.. Full-scale depressurization production testing was carried out at the Mallik site in the winters of 2007 and 2008 (Dallimore et al. 2008b; Yamamoto et al. 2008). A 13-metre zone near the base of the gas hydrate stability field was chosen for production testing, based on reservoir simulations that suggested this would be themost productive interval. A short production test during the first winter used a downhole electrical submersible pump positioned below the perforation interval. The pump was configured to allow downhole separation of gas and water, with the produced gas flowing to the surface and the residual water re-injected into a deeper perforated zone within the same well. The 2007 test results revealed the mobility of the sand-gas-water mix created when the gas hydrate, which bonds and strengthens the sandy reservoir sediments, is dissociated. While the inflow of sand into the well limited the duration of the 2007 pressure drawdown test, a significant production response was observed during approximately 18 hours of testing. Gas flow rates during the latter part of the test

exceeded 5 000 cubic metres a day. Operational problems encountered in 2007 were overcome in 2008 with the use of sand screens and deployment of a redesigned pump positioned above the perforations. Both gas and water flowed to the surface in the 2008 test. The produced gas was metered at the surface and then flared to the atmosphere. The produced water was re-injected into a lower sedimentary formation via a separate water injection well. A downhole heater was used to prevent gas hydrate formation within the wellbore and production tubing. Sustained gas flows ranging from 2 000 to 4 000 cubic metres a day were maintained throughout the 6.75-day test, and operations proceeded smoothly at three successive drawdown pressures. Water production rates were below 20 cubic metres a day during the testing period. The 2007/08 production test at Mallik can be considered as a proof of concept for gas hydrate production by depressurization of a sand- dominated clastic gas hydrate reservoir. The program successfully used conventional oilfield drilling and well-completion technologies adapted for the unique physical and thermodynamic properties of gas hydrates, and the rates of gas production were promising. Further confirmation of the sustained gas-production rates achievable through depressurization will require production tests of much longer duration. Such tests are currently being planned in both Alaska and offshore Japan.

Figure TB-3.2: The Mallik Gas Hydrate Production Research Well, Mackenzie Delta, Northwest Territories, Canada. (Photo courtesy of the Geological Survey of Canada).

A GLOBAL OUTLOOK ON METHANE GAS HYDRATES 69