Simplified Slurry Design Increases Wellsite Success

Abstract

Summary. Highly flexible cement systems can he designed to accommodate a broad range of well conditions with a relatively small number of additives that are readily available from any cementing service company. Fluid-loss additives can be used with dispersants and small amounts of KCI (when applicable) to design slurries for most primary cementing applications-e.g., cementing in CO, and salt-zone environments and prevention of annular flow after cementing. This paper illustrates how materials and slurry properties that are easily specified can be used to improve wellsite success. Introduction The vast array of cementing additives available to the operator today can be mind-boggling, especially if the user does not have direct access to a cementing laboratory for the design and testing of slurries. What at first may appear as a confusing jumble of cementing systems with different applications can be simplified greatly by use of a few well-chosen additives and good cementing practices. Industry has developed several practices that aid in cement-slurry placement. These includepipe movement (either rotation or reciprocation);centralization;wipers across pay zonesuse of both float shoe and collar;minimum two-joint casing between the float shoe and collar;use of both top and bottom cementing plugs;use of appropriate spacers, washes, or scavenger cement slurries;minimization of surge pressures while pipe is run;good displacement practices (to avoid cement channeling); andoptimum use of mixing equipment for density control. We do not intend to expand upon any of these practices. They were mentioned to emphasize that slurry design cannot substitute for well-engineered placement techniques, nor can good mechanical practices substitute for poor slurry design. We advocate the use of fluid-loss additives, coupled with dispersants and small amounts of KCl, as the basis of sound cement-slurry design, a concept that has applications over a wide range of cementing conditions. This paper discussesthe prevention of annular flow after cementing,cementing in plastic salt environments, andcementing in CO, environments. Prevention of Annular Flow After Cementing Annular flow after cementing occurs when the cement column placed in the annulus fails to contain formation pressures and allows formation fluids (gas, oil, or water) to flow into the wellbore. This is, in fact, a kick that occurs after cement has been pumped into place, a phenomenon that has been studied extensively by a number of investigators. Several mechanisms have been postulated to explain this occurrence, including the following.Dehydration of the cement slurry. Dehydration is caused by insufficient fluid-loss control. If it is severe enough, cement solids will bridge the annulus and prevent transmission of the hydrostatic pressure from the still-fluid cement column above the bridge. If the hydrostatic pressure is reduced to a level less than existing formation pressure below the bridge, formation fluids can enter the wellbore.Free-water pockets and channels. These are caused by a separation of cement solids and mix water and can lead to annular flow after cementing because a continuous water channel can exert only the hydrostatic pressure of the mix water itself (usually 8.33 Ibm/gal [998 kg/m 3 1). If this equivalent pressure is less than the formation pressure, annular flow can occur.Slurry; gelation. This can cause flow after cementing because the attraction between the hydrating cement particles may prevent the hydrostatic pressure of the fluid cement column from being fully transmitted to the exposed formations. In effect, the hydrating cement column "supports some of its own weight." Again, if the effective hydrostatic head of the fluid column is reduced to a level below formation pressure, formation fluids can enter the wellbore. Each of these three mechanisms explains why a cement column with a density greater than that of the drilling-fluid column used to drill the well could fail to contain formation pressures. In reality, these mechanisms are not as discrete and as tightly defined as a simple listing may lead the reader to conclude. All three may coexist during a given cement jobs which mechanism will predominate depends on both the well conditions and the cement-slurry design. Some techniques for predicting the potential for annular flow after cementing have been presented in the literature, but sound slurry design using good fluid-loss additives can alleviate all these potential problems. The Cement Unit of Chevron Services' Drilling Technology Center (DTC) initiated a test program to address the problem areas of flow after cementing. Table 1 summarizes the test results for the majority of fluid-loss additives currently, on the market. These tests are intended to show typical field formulations covering a wide range of bottomhole circulating temperatures (BHCT's). The slurries were designed to have thickening times in the 4- to 5-hour range and slurry densities were designed at 16.4 lbm/gal 1965 kg/m, 3, with the exception of Additive F, which was intend by the particular service to be mixed at a density of 15.6 lbm/gal 1869 kg/m] 3. API Class H cement was used for all slurry designs in Table 1, with all tests performed according to API Spec. guidelines. The data in Table 1 illustrate the concept of using a simple slurry design to combat each of the three mechanisms of annular flow alter cementing. First, as the name implies, the function of a fluid-loss additive is to prevent the uncontrolled dehydration of the cement slurry. While there is general agreement that fluid-loss control is beneficial to the cement slurry, there is some contention as to optimum fluid- loss values. Although such factors as permeability and differential pressure affect dehydration of the cement slurry, good field results have been obtained with a maximum value of 100 ml in 30 minutes for casing jobs and 50 mL in 30 minutes for liner cementations. These values are easily obtained because of the multiple role of the fluid-loss additive. Because these materials are organic, they retard the set of the cement slurry. When sufficient fluid-loss additive is used to obtain the required thickening time, the desired level of fluid-loss control is also obtained. Second, fluid-loss control decreases the volume reduction during slurry hydration. Compare the volume-reduction values in Table 1. with the same values for slurries lacking fluid-loss control in Table 2. The fluid-loss-controlled values are generally an order of magnitude smaller than the noncontrolled values. Third, the fluid-loss additive controls free water. The concentration of fluid-loss additive that provides thickening time and hydration control will also control water separation, provided that reasonable cement/water ratios are used. Zero free water under downhole conditions of temperature and pressure is the goal when the cement slurry is designed. Fourth, and finally, cement slurries designed with fluid-loss additives control slurry gelation. Slurries that rely on fluid-loss ad ditives and dispersants exhibit a "right-angle set" as opposed to a "gel set." These two concepts are illustrated by the strip charts in Fig. 1 . The gel set is characterized by a slow gain in consistency until the final pumping time is reached. By contrast, the right- angleset slurry remains thin until close to the end of the thickening time period and then thickens rapidly to final consistency. Also important is the static gel-strength development.

Cement Quality-Control Program Shows Substantial Savings

“…For all filler cements, fluid loss is controIled below 500 mL. [1][2][3] Copyright 1990 Society of Petroleum Engineers SPE Production Engineering, August 1990…”

“…All cement slurries must have less than a trace (0.5%) of free water with all tests conducted at a 45° angle. 1,2 Pumping Time. When possible, minimum pump times are achieved without retarders, especially lignosulfate retarders.…”

“…When a slightly longer pumping time is needed, additional fluid-loss additive or dispersant is used to achieve the desired time. 2 Potassium Chloride. When well conditions allow, all intermediate-and production-casing cementing systems are desifned with 2.0 to 5.0% potassium chloride (by weight of water).…”

Cement Quality Control Program Shows Substantial Savings

“…When a slightly long8r pumping time is needed, additional fluid loss additive or dispersant is used to achieve the desired time. 3 Potassium Chloride -When well conditions allow, all intermediate and production casing cementing systems are designed with 2.0 percent to 5.0 percent potassium chloride (BWOW). 1…”

“…The slurries used were a 1:1 Class G to perlite, 4 percent Bentonite slurry or a Class G, 26 percent Gilsonite® and 4 percent Bentonite slurry. Both of these slurries were mixed in the 12.6 ppg (1.50 g/cm 3 ) to 13.8 ppg (1.65 g/cm 3 ) range. Neither of these cement blends had any contro 1 of filtration or free water and these properties were not monitored.…”