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Tom Brookey, SPE, Acti Systems, Inc. Abstract Horizontal drilling technology has enhanced production through increased ability to expose formation openings. In most cases, these openings are fractured, vugular, or otherwise highly permeable. Many are drilled through low-pressure reservoirs where drilling fluid losses occur and often cause severe formation damage. The use of conventional lost circulation control materials are restricted due to the downhole tools required, and borehole sealing techniques are mostly ineffective in this type of application. A novel new drill-in fluid is being used to drill horizontal and high angle wells through these damage-prone reservoirs. This fluid combines certain surfactants and polymers to create a system of "micro-bubbles" known as aphrons encapsulated in a uniquely viscosified system. These aphrons are non- coalescing and recirculatable so that density reduction is accomplished without expensive air or gas injection. A unique feature of the micro-bubble network, stopping or slowing the entry of fluids into the formation, creates downhole bridging. The unique viscosity builds to create a resistance to movement into and through the zone so that a true noninvasive, at-balance fluid is achieved. Test data shows the enhanced hole cleaning and suspension properties. Case histories show that drilling problems are reduced, mud losses are prevented, and completions are simplified. Natural production was achieved in many cases. No problems with formation damage or inhibited production were seen. P. 645

Aphron drilling fluids are being used globally to drill depleted reservoirs and other underpressured zones. The primary features of these fluids are their unique low-shear rheology and the presence of aphrons, which are specially designed pressure-resistant microbubbles of air. However, how aphron drilling fluids work is not well understood, which limits acceptance of this technology. Recently, a study was undertaken under the auspices of the US Department of Energy (DOE) to gain some understanding of aphron drilling fluids and provide guidance about running these fluids in the filed to optimize performance.Various laboratory techniques were applied to determine the physicochemical properties of aphrons and other components in the fluid and how they affect flow through permeable and fractured media. These included wettability and surface tension, bubble stability, radial and dynamic flow visualization, and fluid displacement tests.One key discovery was that aphrons can survive compression to at least 4,000 psig, whereas conventional bubbles do not survive pressures much higher than a few hundred psig. When drilling fluid migrates into a loss zone under the drill bit, aphrons move faster than the surrounding liquid phase and quickly form a layer of bubbles at the fluid front. The bubble barrier and radial-flow pattern of the fluid rapidly reduce the shear rate and raise the fluid viscosity, severely curtailing fluid invasion.Another key finding is that aphrons show little affinity for each other or for the mineral surfaces of the pore or fracture. Consequently, the seal they form is soft, and their lack of adhesion enables them to be flushed out easily during production. Equally important, the interfacial tension between the base fluid and produced oils or gases is quite low, so that produced fluids do not create a formation-damaging high-viscosity emulsion; instead, they channel through the drilling fluid with relative ease.Depleted wells, which are very expensive to drill underbalanced or with other remediation techniques, have been drilled overbalanced with the aid of aphron drilling fluids.

This paper will provide a case study of the evolution of the drilling systems and techniques used to redevelop a dolomite formation located in the Indian Basin Field, Eddy County, New Mexico, USA.This study will compare data on wells drilled by Kerr-McGee over the past five years in this field where gas productivity ranges from 1000 MCFPD to 10,0000 MCFPD per well from the Upper Pennsylvanian (U. Penn) Cisco and Canyon formation. The following drilling fluid systems utilized to drill 16 subject wells will be discussed:Conventional. Four wells were drilled using conventional water based drilling fluids. When mud loss was encountered in the U. Penn, lost circulation materials (LCM) were used to regain and/or maintain circulation.Drilling "blind" or "dry". Two wells were drilled with this modification of the conventional system. Once returns were lost, an initial attempt was made to regain returns with LCM pills. If this did not restore returns, fresh water was pumped down the drill string to keep the bit face cleaned and drilling would proceed to total depth.Air/Mist. Three wells were drilled using this medium. A string of casing was set prior to drilling into the U. Penn. An air/mist system was then utilized to drill to total depth. The well was completed open hole."Aphron" drilling fluid. This method was similar to the conventional drilling operation, but the drilling fluid system was converted to an "energized air bubble" mud system prior to drilling into the main pay. A total of seven wells were drilled with this system. Background The Indian Basin Upper Pennsylvanian field is a large field covering approximately fifty-seven 640 acre sections located approximately 20 miles from Carlsbad in Eddy County, southeastern New Mexico, USA (Figure 1). It is one of the few fields in New Mexico that has been developed on section spacing. This was feasible due to the extreme permeability of the reservoir rock. The field was discovered by the Ralph Lowe No. 1 Indian Basin test, which was completed on February 5, 1963. The Upper Penn section, occurring at a depth of about 7,500 feet, consists of approximately 85% carbonates. Within the productive limits of the reservoir 80 to 90% of the carbonates are dolomite and the remainder is limestone which appears at the base or top of the section. Shales or marlstones appear throughout the dolomite but probably make up no more than 15% of the total section. The possibility of these shale stringers causing zonal isolation fieldwide has been investigated and is unlikely. Porosity in the dolomite is highly variable ranging from intercrystaline pores to vugs and caverns (Figures 2, 3). The extent and frequency of vugular porosity zones is very difficult to determine, but it is reasonable to expect some continuity of the vugular zones. The average pay thickness is 207 ft and the maximum pay thickness is 319 ft1.

Aphron drilling fluids are being used globally to drill through depleted reservoirs and other under-pressured zones. The primary features of these fluids are their unique low-shear rheology and aphrons (specially designed pressure-resistant microbubbles of air). However, how aphron drilling fluids work is not well understood, which limits acceptance of this technology, along with efforts to optimize the system's performance. Recently a study was undertaken under the auspices of the U.S. Department of Energy to gain some understanding of the workings of aphron drilling fluids. Those results are presented here. Various laboratory techniques were applied to determine the physicochemical properties of aphrons and other components in the fluid and how they affect flow through permeable and fractured media. These included wettability and surface tension, bubble stability, radial and dynamic flow visualization, and fluid displacement tests. One key discovery was that aphrons can survive compression to at least 4000 psig, whereas conventional bubbles do not survive long past a few hundred psig. When drilling fluid migrates into a loss zone under the drill bit, aphrons move faster than the surrounding liquid phase and quickly form a layer of bubbles at the fluid front. At the same time, the shear rate of the fluid continually decreases and the viscosity is rapidly rising. The combination of the bubble layer and the rapidly increasing viscosity of the liquid severely curtails fluid invasion. Another key finding of the study is that aphrons show little affinity for each other or for the mineral surfaces of the pore or fracture; consequently, the seal they form is soft and their lack of adhesion enables them to be flushed out easily during production. Depleted wells which are very expensive to drill underbalanced or with other remediation techniques can now be drilled overbalanced. This study has provided a sound technical basis for the success of aphron drilling fluids and is providing guidance on the way to run these fluids in the field to optimize their performance. Background Aphrons were first described by Sebba1 as unique microspheres with unusual properties. Much of his work was done with microbubbles consisting of air encapsulated in a multi-layer shell created and maintained via chemical equilibrium with various components in the base fluid. Brookey2 described the first use of aphrons in a drilling fluid application. In this case, the microbubbles (as they were then called), were created as a minor phase in a water-based fluid. This system was used as a means of controlling lost circulation and minimizing formation damage in a low- pressure vugular dolomite reef zone. The microbubbles allowed the zone to be drilled to required TD, logged and drill stem-tested; this had not been possible previously. How did the fluid system work? Many at that time thought that density reduction was responsible, since the application resulted in a lower mud weight on surface. The next application was in a fractured dolomite horizontal well, where the bit dropped only a foot and all returns were being lost. In this application, full returns were resumed as soon as the microbubbles reached the bit. Obviously density reduction was not the reason these losses were controlled. This experience led to further research in the area of foams and aerated fluids and to the discovery of Sebba's work with aphrons. Reformulation of the drilling fluid led to increased stability of the aphrons through re-engineering of the multi-layer shell and enhancement of the low-shear-rate viscosity (LSRV), which made the fluid more effective in downhole applications. This new system was applied in South America in an area where six wells were drilled using various fluids and techniques, including underbalanced drilling. Because of severe depletion, lost circulation and borehole instability, none of these wells was successfully drilled to TD. Ramirez3 described the application of aphron technology in this field, which resulted in no drilling fluid losses and excellent wellbore stability even in previously troublesome shale sections. Conditions were so favorable that coring was done with over 90% recovery on the first well. Extensive wire-line logging was carried out with no problems. Even cementing was highly successful, with full returns throughout, though severe cementing problems had been the norm. After drilling the first three wells in this field, the operator was able to eliminate the intermediate string and drill from surface casing to TD successfully.