Solids-Free Brine-in-Oil Emulsions for Well Completion

Abstract

Summary. An invert emulsion fluid composed of brine (e.g., NaCl, NaBr, CaCl2, CaBr2, and ZnBr2) emulsified into hydrocarbon oil (e.g., diesel, crude, or mineral) was formulated with a mixture of nonionic surfactant emulsifiers. Laboratory investigations were conducted to define the emulsion characteristics and to develop methods for controlling the fluid's rheological properties and emulsion stability at elevated temperatures. This system has oil as the external phase, and the stability of the emulsion in most cases is linked to the homogeneity and fineness of the brine-dispersed droplets. The emulsion stability is also related to its viscosity and to the strength of the interfacial film formed by the emulsifiers that coat the brine droplets. For packer-fluid applications, the solids-free invert emulsions offer several advantages over conventional oil- and water-based muds. For example, the fluid is virtually nonconductive, providing greater corrosion protection than conventional water-based muds and clear brines. It can also provide formation protection characteristics and can solve such problems as clay swelling and solids invasion. This new completion fluid is ideal for perforating, drilling in, under-reaming, and gravel packing. The emulsion can be filtered through a less-than-10- m cartridge filter and can be reclaimed and used again. This new completion fluid was used successfully in the field as a packer fluid and as a perforating fluid for testing a reservoir that was drilled with conventional oil-based mud. Introduction Oil-based muds are nonreactive fluids used for drilling and completion operations instead of aqueous fluids, which create similar conditions to the depositional environment of the rocks penetrated and often fail to stabilize the hole. Oil-based muds are being used to drill stable, in-gauge holes through water-sensitive shales and salt sections. They provide superior lubricating qualities over water-based muds and have proved to be less damaging to water-sensitive producing formations in many fields. Also, oil-based muds do not readily conduct an electric current. Thus, corrosion reactions on metal surfaces are not likely to occur, which is why oil-based muds are the preferred packer fluids, especially for high-pressure/high-temperature (HPHT) wells. Conventional oil-based muds usually contain a large quantity of solids (e.g., organophilic clays and barite). These added solids are necessary to develop the required suspension for the weighting material and to form internal bridging in the rock pores to build a wall cake needed to stabilize the wellbore. One of the most expensive workover operations is the recovery of stuck tubing and packers in settled mud solids. High-density conventional oil-based muds are not stable suspensions when used as packer fluids. Downhole tubing or packer leaks cause mud contamination with the produced oil or gas (gas is highly soluble in oil-based muds). The annulus pressure starts building up and must be bled, which allows more oil or gas influx into the annulus. This typical sequence of events leads to the destruction of the initial suspension properties of the mud and allows the mud solids and weighting material (barite) to settle on top of the packer and around the tubing. Several instances where conventional high-density oil-based packer fluids failed in such a manner have been seen in the U.S. gulf coast and internationally. Subsequent wash-over and fishing operations resulted in numerous days lost and hundreds of thousands of dollars in cost. Drilling water-sensitive pay zones with oil-based muds that have all-oil filtrates is usually recommended. For example, reservoir rocks containing volcanic ash and/or smectite or mixed-layer clays could be permanently damaged if they come in contact with any aqueous fluids. In many cases, these types of reservoirs require gravel packing to minimize sand production problems. Viscosified clean oil has proved to be the most suitable gravel-packing fluid for low-pressure reservoirs. Unfortunately, no oil-soluble material is available to increase the oil's density. For high-pressure reservoirs, either high-density brines or conventional oil-based muds with solids must be used for gravel packing, which causes irreparable formation damage and loss of the zone productivity in some cases. The obvious advantage of perforating, underreaming, and possibly drilling in with solids-free invert emulsions triggered our interest in developing simple methods for formulating this type of completion fluid to control formation damage. Our study objectives wereto determine the optimum oil/brine ratios and the required concentrations of the emulsifier to produce the most stable emulsion at temperatures up to 400F [204C],to develop temperature/viscosity profiles,to select the most suitable oil-soluble polymers for viscosity and rheology control,to define the emulsion contact angle against quartz surfaces and the residual damage in terms of return permeability for Berea sandstone cores,to determine the feasibility of using sized acid- and water-soluble materials for the temporary bridging of high-permeability reservoirs,to study the corrosiveness of the emulsion andto define the filterability of the emulsion.

Development of an Oil-Based Gravel-Pack Carrier Fluid

“…Brine-in-oil emulsions have been previously proposed and reported as having been used as packer fluids 11 . In this paper, we present the development of an oil-based gravel pack carrier fluid.…”

New Approach for Completion Applications With Higher Internal Phase Inverts Utilizing Innovative Surfactants

“…In their study, three emulsifiers were required to provide a stable emulsion. Ezzat and Blatell (1989) documented formulations at 40/60 to 50/50 OWR. However these utilized zinc bromide and a polymerized surfactant that necessitated use at higher concentrations when the brine or water phase increased to 60 vol%.…”

“…Nicora et al (2001) reported a more traditional oil/water range of 65/35 to 85/15 as this ratio in the field is often dictated by the concentration of solids from weighting agents, clay and formation which can reach 35 to 40 vol% of total solids. At this concentration, Nicora et al reported unacceptable rheology. In spite of many attempts to develop high internal phase ratio (HIPR) OBMs for OWRs (20/80 to 50/50) with higher water content in the internal phase, the use or success has been very limited (Daynes et al 1987;Ezzat and Blatell 1989;Nicora et al 2001). The primary focus in the development of invert systems has been on the choices of external phase, internal phase, weight materials and other solids content (Nicora et al 2001) while maintaining the drilling performance characteristics expected with OBM.…”