Home - Slot Drill Development

The Slot Drill is a new method of stimulating oil and gas wells. Instead of random hydro fracture methods that produce thin cracks in the rock, Slot Drill methods mechanically cut a substantial slot perpendicular to the well bore up to a hundred feet deep all along the production zone.  These slots can have up to a half million square feet of surface area in the producing zone.  Carter Technologies Co under contract to RPSEA, (Research Partnership to Secure Energy for America) created Slot Drill to be a stimulation method that is more broadly effective  than fracturing, is environmentally safer, uses  less water, and reduces cost.

 

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 the Slot-Drill method, which showed the greatest cost advantage over fracturing, 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 early 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 project team has now developed a Phase 2 proposal to take the leading concept to the engineering phase, cutting slots in a full scale test well, in cooperation with one or more production or service companies and additional Department of Energy funding. 

 

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. The project is currently awaiting funding to perform a demonstration in a test well and locate a suitable partner to perform field tests. 

 

 

Results

The Slot-Drill system 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.

 

 

The Slot-Drill is an advanced cable saw method that operates like a down hole hacksaw.   The abrasive cutting element be 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 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.  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. 

 

This technology is owned by Carter Technologies Co. and is available for commercial license. For more information contact Ernie Carter at cartertech@prodigy.net or call 281-495-2603