Foam cementing has been widely used for offshore applications where the formation has low fracture gradients, and shallow water flow may occur. One challenge of foam cementing is to ensure that the foamed slurry is stable at the desired foam qualities with low permeability. Therefore, stabilizers are commonly applied along with the foaming agent. This paper describes the comparison testing and several offshore applications of a new liquid foam stabilizer that improves logistics and flexibility on the rig, and prevents excessive viscosity and foaming during mixing. The new liquid foam stabilizer was tested in the laboratory to compare its performance with that of a dry stabilizer used previously in the field. The stabilizer was then used in several offshore wells to assess its performance in real-world operations. In particular, one problem with dry foam stabilizers is that they can promote excessive foaming during mixing, which leads to pump cavitation if not controlled with a defoaming agent. The field operations were arranged to test a new operational procedure for use with the liquid foam stabilizer to avoid cavitation without a defoamer. The laboratory testing determined that the new solids-free liquid foam stabilizer disperses easily in the cement slurry. The new stabilizer was also found to improve the stability of the foam and the density distribution of the set foam cement compared with no stabilizer. Furthermore, several case histories will demonstrate that the combination of the liquid additive and the new operational procedure improved the slurry mixability without any pump cavitation issues. The new liquid foam stabilizer has more predictable and reliable response than conventional dry stabilizers, and the case histories will demonstrate how its use improves cementing flexibility and logistics.

Water-soluble cellulose based polymers such as hydroxyethyl cellulose (HEC), methylhydroxyethyl cellulose (MHEC), and carboxymethylhydroxyethyl cellulose (CMHEC) are commonly used additives to control important cement slurry properties like fluid loss, free fluid, and slurry stability. They typically hydrate quickly after contact with the mix water resulting in instant slurry viscosification and potential mixing difficulties especially at higher loadings. Adding cement dispersants to the cementing system design can facilitate slurry mixing but this does not always work out as expected. Sometimes incompatibilities between the dispersant and the other additives result in even more difficult slurry mixing. Furthermore the addition of dispersants to compensate mixing problems can enhance thermal thinning leading to severe settling issues once the slurry is exposed to the elevated downhole temperatures in the wellbore. To overcome these challenges, the use of large particle size cement additives to lower the surface mixing viscosity and the effect of important slurry properties were studied. The cement fluid loss, free fluid control, and slurry rheology performance of large particle size polymeric additives were evaluated in cement slurries. Different particle sizes for polymeric additive were tested in assorted cement densities and temperatures. Cement testing procedures, lab test results, and a case history is presented and discussed. The study demonstrates that the hydration of the cellulosic polymers is delayed as particle size of polymeric additives increases. It could be concluded that the use of larger size cellulosic polymers greatly facilitates slurry mixing, requires less mixing energy or reduces slurry mixing time, and results in a lower initial slurry rheology without negatively affecting other slurry properties (in some cases the fluid loss performance was even significantly improved).

Typical additives used to help suspend cement slurries at high temperatures include either biopolymers, such as polysaccharides, or synthetic polymers. The biopolymers typically degrade at elevated temperatures while the synthetic polymers tend to severely thermal thin leading to inadequate performance from either. This work describes an additive to improve cement slurry stability at higher temperature ranges in horizontal well. A modified thickening time test, commonly referred to as a dynamic settling test, was used to demonstrate the stability of cement slurries at high temperatures. Free fluid tests were also performed where the cement slurries were exposed to downhole conditions prior to evaluating their stability. Ambient rheology profiles were examined to assure that the slurries can be mixed, pumped, and placed without excessive pressures. The effectiveness of this novel additive on fluid loss control is also exhibited by performing stirred fluid loss tests at elevated temperatures. Cement slurry stability is especially difficult to achieve in long, horizontal well cementing, where the slurry is exposed to high temperatures over long periods. The novel cementing additive presented here can help simplify otherwise complex cement slurry designs, which are needed to help meet all operational requirements. In particular, the material helps eliminate the need for excessive viscosifying agents in combination with large quantities of dispersant in order to maintain slurry stability at higher temperatures. The paper compares the effectiveness on cement slurry properties such as stability, free fluid, rheology, and fluid loss control. The tests were conducted from 200 °F to 400 °F in the laboratory at varying concentrations and cement densities. The problems, current solutions, newly developed solution and case history will be discussed in the paper. The work detailed shows that the new additive improves high-temperature cement slurry stability without high surface rheology and the associated mixing/pumping challenges. By providing this stability for horizontal wells, the cement slurry will prevent undesirable consequences which could result in remedial work, poor fracture treatment, communication between zones, and millions of dollars spent in additional completion costs and lost production.

Insufficient wellbore cleaning prior the cementing job is considered to be the biggest single factor leading to poor zonal isolation results. A mud-spacer-cement program with suitable fluid needs to be carefully engineered for the given wellbore conditions to improve cementing quality. We discuss optimum spacer design features which are critical for the successful cementation of deep deviated HPHT wells containing heavy oil based muds and review a simulated scenario. Advanced lab test methodologies beyond industry standards are utilized to model more accurately the given complex downhole conditions. A simulated >20,000 ft highly deviated wellbore was characterized by HPHT bottomhole conditions and the rheological performance of the cement spacer was critical to job success. The well needed a stable cement spacer that would not settle-out on the low side of the >14,000 ft horizontal section, which would potentially put the well at risk. The 16.17 ppg mud required an even higher-density spacer system to clean it effectively. But conventional high-density spacer systems only compound the settling challenge and the well's anticipated bottomhole temperature of 350°F was expected to compromise any additives that might stabilize the fluid systems. Therefore a lab study about spacer stability was performed using a HPHT rheometer and the dynamic settling test – an industry standard which was actually established for cement slurries but not for spacer fluids. We found that a conventional spacer failed at 350°F by showing a rapid decline in rheology to almost zero viscosity and severe settling. To overcome the settling issue, provide stability, and maintain a sufficiently high rheology profile at given 350°F, we re-designed the spacer by using a modified biopolymer which shows a delayed hydration and viscosification over time successfully counteracting the destructive thermal effects. The mud-spacer-cement fluid train was eventually optimized showing good fluid compatibility and maintaining within the narrow, 1.6 ppg margin that separated the pore pressure from the fracture gradient. The cementing job was designed using an advanced fluid displacement software, which predicted high mud removal efficiency under these challenging conditions. In order to enable proper mud displacement, the Friction Hierarchy—a key design factor that is often difficult to achieve under the extreme HPHT well conditions—was achieved with the new spacer concept.

Mechanical performance of cement significantly affects cement sheath integrity and long-term zonal isolation, especially in complex high-temperature, high-pressure (HTHP) wells drilled under challenging operational conditions. A majority of cement sheath mechanical failures are believed to occur in tension rather than compression, which has led to increased focus on cement tensile strength. However, most technical literature related to cement tensile strength refers to testing at ambient temperature and pressure, which does not represent actual downhole conditions. In this study, an HTHP tensile strength testing device is used to measure cement tensile strength in situ at simulated downhole conditions, and these data are compared with measurements gathered at ambient conditions. In this work, class H cement was mixed with mechanical enhancers, an anti-settling agent, and an anti-foaming agent at 16 to 16.4 ppg. Several mechanical enhancers, including polymeric and fiber additives, were investigated. The cement samples were cured inside the HTHP tensiometer at downhole conditions, and the tensile strength was measured in situ at the same curing conditions. The in-situ tensile strength results were compared with results from the traditionally used uniaxial method at ambient conditions. The difference between the tensile strength tested in situ and at ambient conditions varied depending on the type of mechanical enhancer. For example, a significant increase in in-situ tensile strength was observed for cement with polymeric additives. This study demonstrates that testing environment significantly affects the measured mechanical properties of cement, and conventional ambient measurements may not accurately reflect downhole performance. It draws attention to industrywide concerns about developing cement systems with mechanical properties optimized for HTHP and other challenging downhole environments.