Optimizing cement systems for specific well conditions

Reducing remedial and intervention costs

Phil Pattillo - BP America

Kris Ravi, BR Reddy, Dennis Gray - Halliburton

Sustained casing pressure (SCP) is a challenge faced by the oil and gas industry during the life cycle of a well. From the cementing point of view, the SCP could be caused by inadequate drilling fluid displacement and/or the failure of cement sheath to withstand well operational loads. The ideal is to design and deploy the primary cementing jobs so that the cement slurry is placed in the entire annulus circumference and the cement sheath properties are fit for purpose to withstand downhole chemicals and the anticipated stresses from well operations.

The main purpose of a primary cementing job is to provide effective zonal isolation for the life of the well so that oil and gas can be produced safely and economically. To achieve this objective, the drilling fluid from both the wide and narrow segments of the annular cross section should be removed and the entire annular cross section filled with a competent cement system.

However, from a cementing perspective, a path for fluid migration could be created:

• If drilling fluid is not effectively displaced
• If cement slurry is not placed in the entire annular circumference
• If the cement sheath fails during well operations.

A cement system should meet both the short-term and long-term requirements imposed by the operational regime of the well. However, the industry has generally concentrated on the short-term properties that are applicable during the cement slurry stage. This is necessary for effective cement slurry mixing and placement, but it is not sufficient for long-term well integrity. It has been common practice to assume that high compressive strengths are adequate indications of long-term performance. However, the long-term integrity of a cement system depends on its shrinkage and expansion characteristics, mechanical properties such as Young’s Modulus, tensile strength, and resistance to factors present in the downhole environment, such as temperature and chemicals.

Table 1. Short-term and long-term properties recommended of cement systemShort-term: cement slurryLong-term: cement sheathEnvironmentally friendlyThermally and chemically stable under downhole conditions of pressure and temperatureMixable at surfaceResist shrinkageStableOptimum mechanical properties to withstand stresses from various downhole operations and provide zonal isolation for the life of the wellOptimum thickening time and densityOptimum strength development and fluid loss100% placement in the annulusResist fluid influxResist shrinkage during hydrationThe industry acknowledges that a cement system should meet both the short-term and long-term requirements imposed by the operational regime of the well.

Zonal isolation failure

After the cement slurry is placed in the annulus, if there is no immediate migration of formation fluid to the surface, it is likely that short-term properties, such as density, rate-of-strength development, and fluid loss of the cement slurry were designed satisfactorily. From a long-term perspective, however, after operations such as completions, pressure testing, hydraulic stimulation, and production, the cement sheath could lose its ability to provide zonal isolation. A well’s loss of zonal isolation could create a path for formation fluids to enter the cemented annulus, which could pressurize the well. If this happens then remedial cementing may be needed before the well can continue normal hydrocarbon production.

Failure of the cement sheath is most often attributed to stresses induced by pressure and temperature changes resulting from operations during the well’s economic life. Examples of wells in which the cement sheath may be subjected to significant stress levels are:

• High-pressure and/or high-temperature (HP, HT, and HPHT) wells
• Deepwater wells
• Gas-storage wells
• Wells penetrating weak, unconsolidated formations
• Steam-injection wells.

Cement sheath integrity

Historically, the effect of well operations on the integrity of the cement sheath has not been taken into account when selecting the cement slurry. Only recently has the subject received the deserved industry attention.

In 1990, K. J. Goodwin conducted an experimental study to evaluate the performance of different cement systems subjected to various pressure and temperature changes. His study demonstrated that stiff cement sheaths or cement sheaths that possess a high Young’s modulus are more susceptible to damage caused by pressure and/or temperature change. His work also pointed out the importance of the Young’s modulus of a sealant. In 1996, O. G. Benge recognized the unique elastic or resilient behavior of a foamed cement system and successfully implemented such systems in the Gulf of Mexico, particularly in HPHT applications.

This experimental work was then followed by mathematical studies of the effect of stresses on a cement sheath. M. Bosma simulated the mechanical response of a cement sheath based on finite element analysis (FEA). M. J. Thiercelin applied analytical procedures to study the effect of cement sheath mechanical properties, assuming fully-bonded or non-bonded sheath interfaces. Bosma modeled debonding, cracking, and plastic deformation for cement sheath failure modes and simulated the effect of cement sheath shrinkage and expansion. Several other contributions have appeared since those studies were published.

Major consequences of damage to the cement sheath, such as annulus pressure or damage to the casing, could force well shutdown or result in high remedial costs. Other consequences of damage to the cement sheath, such as loss of hydrocarbon production, production of unwanted fluids (e.g. water), and wellhead movement, could negatively affect the normal operations of oil and gas assets. Therefore, the industry should consider long-term integrity of the cement sheath during the early stages of well construction and design for uninterrupted, safe, and economic production of hydrocarbons.

Cement slurry hydration

A cement slurry undergoes a hydration reaction in the presence of water that includes the main constituents of Portland cement; C3S, C2S, C3A, and C4AF. When Portland cement is mixed with water, the main products that contribute to the long term mechanical properties are CSH gel and calcium hydroxide. The volume of the products formed is less than the volume occupied by the reactants. This phenomenon is termedhydration volume shrinkage, with the total volume shrinkage being classified as bulk (external shrinkage) and contraction of pores (internal shrinkage). The construction industry refers to drying shrinkage as the shrinkage observed when the cement sheath is cured in the absence of water, after curing in the presence of water for a certain number of days. Although drying shrinkage does not apply directly to oil well cementing, the findings can increase overall understanding of cement slurry behavior.