There are several different types of drilling fluids, based on both their composition and use. Selecting the correct type of fluid for the specific conditions is an important part of successful drilling operations. Water-based fluids (WBFs) are the most widely used systems, and are considered less expensive than oil-based fluids (OBFs) or synthetic-based fluids (SBFs). The OBFs and SBFs--also known as invert-emulsion systems--have an oil or synthetic base fluid as the continuous(or external) phase, and brine as the internal phase. Invert-emulsion systems have a higher cost per unit than most water-based fluids, so they often are selected when well conditions call for reliable shale inhibition and/or excellent lubricity.
Proper placement of conformance improvement gels is key to achieving the desired results within the reservoir. The flow properties of a gelant or gel as it is being placed are important parameters. To date, for all known gelant solutions used in conformance improvement treatments (including polymer gelant solutions), these gelant solutions place themselves in all matrix-rock geological strata according to Darcy flow considerations and do so without any special selective placement in only the high-permeability strata and flow paths. Any placement of gel into, and the associated permeability reduction of, a low-permeability and/or high oil saturation strata in the near-wellbore region surrounding a radial-flow matrix-rock-reservoir well will almost always be counter productive to improving the conformance of that well. For a gel treatment to be successful when treating vertical conformance problems in matrix-rock reservoirs in which there is fluid flow and pressure crossflow communication between geological strata of differing permeabilities, the gel treatment must be selectively placed deeply in the reservoir.
Many approaches to estimating permeability exist. Recognizing the importance of rock type, various petrophysical (grain size, surface area, and pore size) models have been developed. This page explores techniques for applying well logs and other data to the problem of predicting permeability [k or log(k)] in uncored wells. If the rock formation of interest has a fairly uniform grain composition and a common diagenetic history, then log(k)-Φ patterns are simple, straightforward statistical prediction techniques can be used, and reservoir zonation is not required. However, if a field encompasses several lithologies, perhaps with varying diagenetic imprints resulting from varying mineral composition and fluid flow histories, then the log(k)-Φ patterns are scattered, and reservoir zonation is required before predictive techniques can be applied.
When conducting a polymer waterflood, a high-molecular-weight and viscosity-enhancing polymer is added to the water of the waterflood to decrease the mobility of the flood water and, as a consequence, improve the sweep efficiency of the waterflood. The primary purpose of adding polymer to most polymer waterfloods is to increase the viscosity of the flood water; however, polymer addition to the flood water in many instances also imparts a secondary permeability-reduction component. Polymer waterflooding is normally applied when the waterflood mobility ratio is high or the heterogeneity of the reservoir is high. Figure 1 shows the polymer waterflooding process. The method shown requires a preflush to condition the reservoir, the injection of a polymer solution for mobility control to minimize channeling, and a driving fluid (water) to move the polymer solution and resulting oil bank to production wells. Mobility ratio is improved and flow through more permeable channels is reduced, resulting in increased volumetric sweep. Waterflooding promotes improved sweep efficiency by improving the mobility ratio.
This article describes the chemical make-up and application of the types of gels most commonly used in conformance improvement. It also discusses the ways in which these gels can be classified. Oilfield conformance improvement gels come in a wide range of forms and chemistries. Table 1 provides an overview of various conformance improvement gels. CC/AP gels have an exceptionally robust gel chemistry and are highly insensitive to oilfield and reservoir interferences and environments.
Disproportionate permeability reduction (DPR) is a phenomenon whereby many water-soluble polymers and many polymer gels reduce the permeability to water flow to a greater extent than to oil or gas flow. DPR is also referred to as relative permeability modification (RPM). In Disproportionate permeability reduction, a review is presented of the concepts, applicability, limitations, and desirability of the DPR phenomenon as it applies to conformance improvement water-shutoff (and/or water-reduction) treatments. This article focuses specifically on polymer use for DPR and RPM. As early as 1964, certain polymer flood water soluble polymers were known to impart DPR to water flow in reservoir rock that had been previously flooded with the polymer.
Although conformance-improvement gel treatments have existed for a number of decades, their widespread use has only begun to emerge. Early oilfield gels tended to be stable and function well during testing and evaluation in the laboratory, but failed to be stable and to function downhole as intended because they lacked robust chemistries. Also, because of a lack of modern technology, many reservoir and flooding conformance problems were not understood, correctly depicted, or properly diagnosed. In addition, numerous individuals and organizations tended to make excessive claims about what early oilfield gel technologies could and would do. The success rate of these gel treatments was low and conducting such treatments was considered high risk. As a result, conformance-improvement gel technologies developed a somewhat bad reputation in the industry. Only recently has this reputation begun to improve. The information presented in this chapter can help petroleum engineers evaluate oilfield conformance gels and their field application on the basis of well-founded-scientific, sound-engineering, and field-performance merits.
Early application of polymers for use during oilfield conformance improvement operations was focused on improving volumetric sweep efficiency of waterfloods. More recently, polymers have been used extensively in disproportionate permeability reduction (DPR) and relative permeability modification (RPM) treatments for water shutoff and in conformance improvement polymer-gel treatments. This page discusses polymers used in oilfield operations and how they contribute to conformance improvement. Polymer molecules are the resultant chemical specie when a large number of relatively small and repeating molecular entities, called monomers, are joined together chemically. The chemical process of joining together the monomers and forming polymer molecules is referred to as the polymerization reaction process.