This paper demonstrates how simple instrumentation can be used for evaluating the wett ability of cementing spacers and preflushes in a laboratory. With detailed knowledge of a solution's "apparent" wett ability, service companies and operators can more effectively choose the surfactants required for displacing nonaqueous drilling fluids and leaving the casing "water-wet." Water-wet casing allows the cement to adhere more easily to the surface, reducing the potential for remedial zonal isolation treatments.
A new laboratory device has been developed that more precisely assesses wett ability. This device indicates "apparent" wett ability by measuring the electrical activity in the test fluid during the water-wetting process. Oil-external drilling fluids do not conduct electricity; water-external spacers do. As the oil-wet surface of the test container becomes water-wet, the device registers electrical activity and a meter displays the "apparent" wett ability in dimensionless units called Hogans (Hn).
Introduction
Historically, technicians have evaluated wett ability by dipping a glass rod into the drilling fluid and observing the fluid's behaviour on the surface of the rod. The rod is then dipped in the spacer solution and rinsed with water until it is free of the fluid and any oil film. At that point, the surface of the rod is considered to be water-wet. Several variations of gravimetric evaluations also exist where a test coupon is weighed before and after it has soaked for a specified period in various fluids.
Because these methods are highly subjective, quality control is virtually impossible. Factors such as temperature variation, hardness, amount of water, and use of agitation can affect test results, and tests involving the same drilling fluid and spacers are subject to varied interpretation. Additionally, gravimetric methods do not account for the effects of high-yield-point fluids that can adhere to the coupons, rendering any weight indication of drilling-fluid removal or water wett ability useless.
Pure drilling fluid and pure spacer do not accurately represent the drilling fluid/spacer mixture that occurs in the down hole mixing zone. Compared with actual field results, the test results obtained with the pure forms can be exaggerated. The new apparent wett ability apparatus was developed in response to the demand for greater precision in wett ability assessment. This paper presents case-history accounts of wett ability measurement in the field, and shows how using instrumentation to determine apparent wett ability saves time and cost of materials. Specifically, the discussion focuses on two locations, south Texas and the Gulf of Mexico (GOM). The paper does not deal directly with the rheological aspects of displacement. However, because surfactants affect the rheology of the mixing interface, the surfactant packages designed through testing result in greater compatibility and displacement efficiency.
Wett Ability Transition Through the Drilling Fluid/Spacer Interface Volume
Understanding the relationship between emulsions and drilling fluids/spacers is a prerequisite for designing a surfactant package that will displace a nonaqueous drilling fluid with a cement spacer. Synthetic, mineral, and oil-based drilling fluids are oleaginous (oil-external) fluids. These fluids consist of an oil or synthetic continuous phase and an internal aqueous phase, usually containing salts such as calcium chloride (CaCl2) in concentrations up to saturation (Fig. 1, Page 6).
As an aqueous spacer displaces the oleaginous fluid in the wellbore, both fluids come into contact in the intermixing zone. Throughout the intermixing zone, the oleaginous emulsion will absorb water until the water droplets become so large that the external oil layer can no longer contain them (Fig. 2, Page 6). At this point, the emulsion breaks.1
If the aqueous fluid contains no surfactants or contains inappropriate types or concentrations of surfactants, the break in the emulsion can result in phase separation, the settling of solids, or a spike in viscosity. All of these results can be detrimental to the displacement process.
