To continue our disquisition about replacing water in hydraulic fracturing operations, let’s look at a couple of long shots in that race. Before we do, here’s a plug for an SPE Forum, set for later this year, titled “Waterless Fracturing: Reducing Fresh Water Use for Reservoir Stimulation in a Future Water-Constrained World,” which underscores the problem.
Mechanical cutting of the shale formation
Perhaps the most adventurous technique, a 2010 patent presented a method to remove mass from a formation between two connected wellbores using a flexible cutting cable. According to this idea, two wellbores are drilled and connected; a cutting cable is inserted into the first well and fished out from the second; finally, the cable is repeatedly pulled back and forth. This sawing action removes formation material between the wellbores to form an opening in the shape of a plane. Earlier patents proposed methods to remove minerals, such as coal, from seams using a chain cutter that is pulled through the seam, for instance from a tunnel drilled in a “U” shape.
Drawing on these ideas, an interesting project (“Novel Concepts for Unconventional Gas Development in Shales, Tight Sands and Coalbeds”) was launched and funded by the National Energy Technology Laboratory (NETL) and performed by Carter Technologies between 2008 and 2009.
The project objective was to develop an alternative method of stimulation to increase the net production of gas from shale while reducing the amount of water required. Over a dozen new concepts were evaluated, including one promising method (called slot drill) that appears to be able to cut 100-ft deep slots all along a 2,500-ft horizontal well. According to the project’s final report, the method appears to have a low capital cost and be sufficiently robust to withstand the rigors of the downhole drilling environment.
Downhole hacksaw
The mechanically simple slot drill operates like a downhole hacksaw. A well is drilled to depth in the target formation and a casing is cemented. The hole is then directionally drilled to curve back upward in the shape of a “J” within the producing formation. The drillstring is retrieved back to the surface, and an abrasive cable is attached to the tip of the drill pipe. A winch on the rig holds a specific tension on the cable, as the pipe is lowered back into the hole under its own weight. Such tension causes the cable to hug the inside radius of the curved hole. The cable is moved back and forth, and this motion cuts a pathway upward from the hole on each downward stroke. The cutting force at any point is a function of local cable tension and radius of curvature, so the shape of the cut may be tailored to some extent. The cut is nominally upward along a vertical path, but it can also be made to turn horizontally.
An operation may last from two to five days, depending on the desired depth of cut and the hardness of the rock. Drilling fluid is circulated through the drill pipe to flush the cuttings back to the surface. The abraded cuttings are very small particles and circulate out easily. A special tool is also used to allow a standard blowout preventer to seal on the cable and drill pipe.
The system should be able to cut a 100-ft deep vertical slot upward from the horizontal lateral in a shale formation, and the cut length could exceed 2,500 ft. This system would operate in a blind hole from a conventional drilling rig and is powered by the rig. The only special equipment required is a constant tension winch, and a downhole tool, that connect the abrasive cable to the drill pipe.
Fully developed slot-drilled wells could result in much greater total recovery from a given lease acreage, thus increasing the total proven reserves. These slots also may reduce production decline, reduce the effects of formation damage, and allow a larger percentage of the gas in place to be recovered.
According to the project results, reservoir simulations indicated that the slot, alone, may increase well flowrates significantly, compared to current fracturing treatments. Unlike hydraulic fracturing, the location of a slot can be selected and precisely placed, is much thicker, and has near unlimited conductivity.
Enhanced bacterial methanogenesis
A significant part of organic-rich shales has not undergone a sufficient burial to generate the pressure and temperature conditions necessary for the complete transformation of the organic matter into oil or coal. These immature source rocks may represent a huge fossil carbon resource.
Microbiologically assisted methanization of the organic matter is known to occur in shales from field data showing the natural accumulation of biogenic methane in several sedimentary basins. Among other promising research, Schlumberger-Doll Research funded a project (“Toward microbially-enhanced shale gas production”) to genetically profile the bacterial communities present in formation water of three gas-producing Antrim shale wells. Incubation experiments were established by adding different substrates to aliquots of these waters in an effort to stimulate the microbial methane generation. Increases in direct methane production were obtained.
Results of a study conducted on shales from Abbey field in Western Canada showed that biogenic shale gas generation accounted for about 12% of the total gas produced. According to the authors, this is a significant percentage of the total gas production and suggests great potential to enhance methanogenesis within these reservoirs.
The bottom line appears to be this: Methanogens can produce a significant amount of methane without any stimulation. And, as anyone with a toolbox knows, sometimes a hacksaw is the perfect tool.
石油圈
