Tip Screenout Fracturing: A Technique for Soft, Unstable Formations

Abstract

Summary. Several special stimulation techniques have been proposed for soft, unstable formations. One example is a combination of acid and proppant, which leaves the acid channels full of sand. This paper proppant, which leaves the acid channels full of sand. This paper discusses an alternative completion technique that involves only normal sand fracture procedures and is effective where acid is ineffectual or undesirable. Because the procedure is aimed primarily at creating short, high- conductivity fractures in soft formations, its use can be extended to any formation where such a treatment is practical and economically feasible. The fracture design is based on intentionally screening out the tip of the fracture with sand and then continuing to pump slurry to increase the fracture width and to pack the fracture with proppant to obtain high conductivity. Because this involves severe risk of a premature screenout, and because failure to achieve the tip screenout will not yield the desired stimulation, special prefracture tests are required to determine fracture design variables. The implementation and analysis of these tests are discussed, along with the theory behind the fracture design schedule. This theory is then compared with fracture pressure data and production performance from two wells completed in an Upper Cretaceous production performance from two wells completed in an Upper Cretaceous chalk formation in the Norwegian Sector of the North Sea. Introduction From time to time, the oil industry is faced with completing wells in very soft, unstable formations, such as chalk. Such rocks create unusual situations regarding drilling, completion, and production practices. Many normal stimulation techniques are rendered ineffective by the mechanical properties of the formation. For example, acid-etched channels quickly collapse and close as the pore pressure is reduced, or the proppant from a normal sand pressure is reduced, or the proppant from a normal sand fracture quickly embeds in the formation, leaving little or no fracture conductivity. In some formations, perforating the pay section may even be undesirable because of perforating the pay section may even be undesirable because of solids production and possible casing collapse. In this situation, fracturing is required to create a highly conductive flow path from the pay zone to perforations in a more stable formation. A unique well completion and stimulation program has been applied to a chalk reservoir located in the Norwegian Sector of the North Sea-unique in the sense that the hydraulic fracturing stimulations attempt to achieve on purpose what often occurs accidentally. That is, the purpose what often occurs accidentally. That is, the success of the stimulation depends on the creation of a controlled screenout to achieve enough propped fracture width to ensure lasting fracture conductivity. Problems associated with well completions and stimulations in soft chalk formations have recently been covered thoroughly and will not be discussed here, except to note that both studies concurred that very wide propped fractures with in-situ proppant concentrations on the order of 2 lbm/ft3 (32 kg/m3) are necessary to achieve adequate fracture conductivity. Other investigators have arrived at similar conclusions for another soft formation, diatomaceous earth. For soft chalks, Ref. 2 noted that propped hydraulic fractures are probably the best propped hydraulic fractures are probably the best stimulation technique, but if job size is limited for any reason, normal fracturing stimulation procedures may not achieve an adequate in-situ proppant concentration. For such cases, a procedure was proposed involving a combination of acid fracturing and proppant fracturing where the proppant would leave the wide acid-etched channels proppant would leave the wide acid-etched channels propped open. For diatomaceous earth, Ref. 4 propped open. For diatomaceous earth, Ref. 4 recommended use of very-high-viscosity crosslinked gels to achieve sufficient width for fracture conductivity. For reasons discussed in this paper, these procedures were not applicable to this particular field. Fig. 1 shows a type log of the two main producing zones of the reservoir in question. Early in the exploration program, we determined that stimulation was required in both program, we determined that stimulation was required in both zones to achieve desired producing rates. For the lower zone, the relatively low permeability (1 to 2 md) common to many chalks required stimulation to develop the reserves adequately, while stimulation was required in the higher-capacity upper zone because of wellbore skin problems and possible stress-sensitive permeability. Besides problems and possible stress-sensitive permeability. Besides the need for stimulation, the properties of the upper zone dictated that only oil-based completion fluids be considered. The strength of this zone was found to be very low because of its high porosity (often greater than 45 %); as a result of in-situ oil saturations greater than 90%, any contact with water further degraded this strength. SPEPE p. 95