07122-07

Goal
The objective of this project was to conduct a preliminary evaluation of novel methods for stimulating gas production from low-permeability rocks, including shale, tight sands, and coal bed formations. Specifically, the goal was to assess the feasibility of using a saw to cut a deep slot from a horizontal borehole into the formation at 5,000 to 10,000 foot depths.

Performers
Carter Technologies Company, Sugarland, TX 77478
M-I LLC (a Smith/Schlumberger Company), Houston, TX 77072
University of Oklahoma, Norman, OK 73072
Texas A&M University, College Station, TX 77843

Background
About 40 percent of the natural gas produced in the U.S. comes from unconventional reservoirs, mainly from very low-permeability formations. In order to produce this gas, it is necessary to stimulate each well with hydraulic fracturing. Unfortunately, hydraulic fracturing has some problems associated with it. These problems include: 1) Risk of fracturing into adjacent water zones or high permeability leak zones due to limited control on the placement of the induced fractures; 2) Water resource issues associated with the vast quantities of water required to create the fractures; 3) Fractures are very thin so formation damage due to water sensitive clays and gelling additives may result in the substantial flow restriction within the completed fracture; 4) Production is high after initial fracturing but falls off dramatically after a year or two of production; 5) Only a tenth of the gas in place can be produced due to the rapid decline in production rate; 6) Knowledge of the distribution of natural fractures in the target formation is required to design a treatment that will achieve optimal well performance; 7) Fracturing is expensive often costing more than drilling the well.

This project’s aim was to develop conceptual designs for an alternative method of well stimulation that involves cutting deep slots from the borehole into the adjacent formation to increase the volume of reservoir rock exposed to the well bore and thereby enhance gas production. Carter Technologies created and evaluated many different methods from rotating mills to high-pressure water jets. Carter performed computer modeling using public domain and peer reviewed friction equations to evaluate the feasibility of the slot cutting hardware designs. Published data on drill pipe coefficient of friction along with proprietary data from Carter on the coefficient of friction of cable cutting rock was used in an elaborate model to evaluate force and friction. Concept and design drawings were prepared for the most promising concepts, and these concepts were evaluated and compared. Multiple potentially feasible concepts were considered in parallel throughout the project term with at least one method, which showed the greatest cost advantage over fracturing, to be recommended for further development.

The project partners, including M-I LLC (MI SWACO), University of Oklahoma, and Texas A&M University, reviewed the preferred slot cutting concepts developed by Carter. Fracturing expert, professor Peter Valko of Texas A&M University performed a reservoir production improvement computer analysis using his DVS, (Distributed Volumetric Sources) model. This is a type of volumetric boundary element modeling computer program that evaluates production improvements. This preliminary computer analysis shows that the highly conductive open slots should increase production at least as much as intensive fracturing and perhaps 4 times as much. Dr. Younane N. Abousleiman, Director of the PoroMechnaics Institute at the University of Oklahoma concluded that the planar geometry of the infinitely conductive slot inherently produces a major reduction in formation damage permeability loss effect and should suffer significantly less production decline over time. M-I LLC reviewed the project at various stages for overall viability and cuttings removal and mud circulation dynamics.

The project team planed to develop a Phase 2 proposal to take the leading concept to the engineering phase, in cooperation with one or more production or service companies and additional RPSEA funding, if a feasible slot cutting method could be found. This project was focused on early-stage conceptual development and did not include any laboratory or field work other than an informal bench test confirming the friction model predictions. The deliverable for this project is a Final Report detailing all technical aspects of the project and including design and modeling results and recommendations.

Potential Impacts
Successful development of the proposed formation cutting concept could provide an alternative stimulation method comparable to conventional hydraulic fracturing but at a lower cost and without the huge water resource requirements. Computer models indicate that fully developed Slot-Drilled wells could result in much greater total recovery from a given lease acreage, thus increasing the total proven reserves. Reservoir simulations indicate that the slot alone may increase well flow rates significantly compared to current state of the art fracturing treatments. Unlike hydraulic fracturing, the location of a slot can be selected and precisely placed, is much thicker, and has near unlimited conductivity. If this methodology is developed and applied to suitable unconventional gas reservoirs it could lead to enhanced recovery of gas from these reservoirs, development of reserves in fields that would not otherwise be produced, and accelerated rates of unconventional gas production. Increased domestic gas production would result in increased tax revenues, royalties, and regional economic benefits. Improved recovery for individual wells has the added benefit of decreasing the environmental footprint of a field development program.

Total cost for comparable stimulation benefit is expected to be less than half the cost of current fracturing technology. This cost advantage could make Slot-Drill a potentially be a disruptive technology that causes major changes in the drilling and stimulation service company market.

Accomplishments
A draft Final Report was submitted on February 19, 2009. This document will be made available after it has been reviewed and finalized. The project objectives have been satisfied with at least one method ready to be advanced to an engineering phase. Presentations have been made to a major service company and driller for industry feedback. The results were also presented to RPSEA, and Industry representatives.

Current Status
Though the method appears to be robust and entirely feasible, this was only a preliminary study and significant engineering work and field testing will still be required to bring this method to a sufficiently marketable status that it can be tested in a major shale gas producing area.

The project is currently awaiting funding opportunities to perform a more detailed engineering evaluation of all components and locate a suitable partner to perform field tests. The current economic conditions and market decline may have effectively cut off private sector money for additional research. The team believes that a small scale test is the next logical step in the development so this will be included in our response to the next funding opportunity.

Results
The Slot-Drill is the leading concept resulting from this study. It meets the technical objectives of the project, and appears likely to result in a cost-effective solution. The tool is mechanically simple and robust, and its operation and control appear to be straightforward. Its potential for getting stuck in the hole appears to be low due to the inherently low friction on the pull. It appears that well surface area can be increased by a factor of 50 to 100 times using this tool, and the slot cut is much more likely to access the bulk of the natural fractures and formation layers. A planar slot may suffer far less influx degradation due to formation damage compared to a wellbore that draws from a radial pattern.

Drawings were prepared to define the geometry and hardware of each of the proposed slot cutting methods, and all the initial concepts were evaluated and compared. Evaluation of each concept consisted of mechanical analysis, thermal conditions analysis, and productivity analysis. In addition, specific practical criteria were considered, including cutting force, cuttings removal, and mechanical wear on drilling systems, equipment and tool costs, and fluid recycling capabilities. In addition, the project team considered some initial concepts for tuned pulse fracturing of a deep slot using in-situ natural gas as the energy source.

The most feasible concept for cutting slots was identified based on the results of analysis and modeling. The concept judged to be most promising for near term development was an abrasive cable saw based tool. This tool was selected over others due to its relatively narrow cut, better cuttings transport characteristics and smaller water resource and energy requirements. Data from the manufacturer of the abrasive materials indicate that cutting speed and durability should be sufficient for the intended one time use of the abrasive cable.

The Slot-Drill is an advanced cable saw method that operates like a down hole hacksaw. The abrasive cutting element is held in tension by a mechanical frame, i.e. the drill pipe, as it is reciprocated along the cut. The force pressing the cable saw into the rock face is generated by the cable tension around the curve of the hole. The system should be able to cut a 100 foot deep vertical slot upward from the horizontal lateral in a gas shale. Modeling indicated that cut length can exceed 2500 feet. This system would operate in a blind hole from a conventional drilling rig and is powered by the drilling rig. The only special equipment required is a marine type constant tension winch and a down hole tool that connect the abrasive cable to the drill pipe. The method requires directional drilling services of industry standard skill. The primary costs of the method are the rig time, the winch, and the consumable abrasive cable materials.

A well is drilled to depth in the target formation and a casing cemented. The hole is then directionally drilled to curve back upward like a “J” within the producing formation. Figure 1 shows a well with two horizontal laterals, one going left and one going right to illustrate two alternate approaches. The one to the right has straight sections with tight curves while the one to the left has a more continuous curve. The drill string is retrieved back to the surface and an abrasive cable is attached to the tip of the drill pipe by a special down hole tool. 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. The cable tension prevents the pipe from rotating and wrapping up the cable on the way into the vertical part of the hole. The cable tension also causes the cable to hug the inside radius of the curved hole while the pipe compressive loading causes it to hug the outside radius of the curve. The friction on the cable around the curve multiplies the initial low cable tension from the winch, increasing exponentially around the curved path. The abrasive cable 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 can also be made to turn horizontally.

The rig may reciprocate the pipe up and down with its 90 foot stroke for 2 to 5 days depending on the desired depth of cut and the hardness of the rock. On the up stroke the cable tension is limited to that provided from the winch so the up stroke performs little cutting. Highest cable tension is inherently at the end of the pipe so the entire cable may be easily pulled out of the hole in the event of breakage. The tool at the end of the pipe can also release the cable to allow it to be pulled out at any time, even if lateral stresses tend to partially close the nominally 1.5 inch wide slot. 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 where they are removed by an MI SWACO type solids control system using hydro-cyclone units. A special tool is also used to allow a standard blow out preventer to seal on the cable and drill pipe. After the slot is complete, the drilling mud may be reversed up the drill pipe by applying annular gas pressure. In unstable formations, the drill pipe may be perforated to become the production string to eliminate the need to trip out of the hole. In very unstable holes a pretreatment of the hole while drilling can stabilize the main hole. Opening a slot of nearly half a million square feet is likely to produce a significant initial flow of gas. Use of a sufficiently heavy salt gel mud to hold the formation in place has the advantage of also controlling gas kicks.

Figure 1. A well with two horizontal laterals, one going left and one going right to illustrate two alternate approaches.

Project Start: 7/24/2008
Project End: 2/19/2009

DOE Contribution: $91,680
Performer Contribution: $22,920

Contact Information:
RPSEA – Kent Perry (kent.perry@gastechnology.org or 847-768-0961)
NETL – Virginia Weyland (Virginia.Weyland@netl.doe.gov or 281-494-2517)
Carter Technologies – Ernest Carter (cartertech@prodigy.net or 281-495-2603)

Additional Information

Final Project Report [PDF]