Today, development of high early compressive strength (CS) is a key performance specification for oilwell cementing. Early strength is important to ensure structural support to the casing and hydraulic/mechanical isolation of downhole intervals. Delays in strength development cause significant amounts of lost time due to the need to Wait-On-Cement. Thus, drilling operations cannot proceed and the rig must sit idle until the cement is deemed hard enough to continue.
The actual development of strength in cement systems is dependent on a number of factors. The type of cementitious material is important due in part to chemistry and in part to granulometry. Slurry density is considered critical - lower density has, traditionally, been associated with lower strength. Temperature is a key parameter and, to a lesser extent, the pressure. Less appreciated, is the influence of the many types of additives that are included in slurry formulations. Correct selection of cement and additives allows slurry tailoring to achieve "a strong cement" that will support mechanical stresses that happen during the well productive life. Since cement is the primary means of isolation, its integrity as a sealing material should be paramount.
Under confining pressure, cement compressive strength is several times higher that the values obtained from an unconfined API test. In fact, the ultimate compressive strength of a wide variety of slurries is similar to that of the surrounding reservoir rock, regardless of initial slurry density. In the long term, more important than compressive strength is the ductility/compliance that set cement has to support down-hole stresses imposed by pressure and temperature oscillations during drilling, work-over and production operations. In the vast majority of well operations, cement failure occurs under tensile loading, not under compression. Poor appreciation of this has led to the routine use of higher density slurries for many applications. Such slurries may be not only unnecessary but also positively harmful, since they can produce more brittle cements that fail under downhole stresses.
This paper describes a method of designing cement slurries using a simulator that models cement setting and strength development. Instead of obsolete rules of thumb, mechanical properties of set cement, formation and casing responses to cyclic stresses during the well life are used to optimise designs for early times and long term. Finally, a few case histories are presented, demonstrating the benefits of this new design methodology to produce cost effective cement slurries for one isolation.
The most important objective of any primary cement job is to provide isolation from producing zones up to the surface and this should be accomplished over the entire life of the well. No fluid movement, either gas or liquid should be possible at any time through the cemented annulus. In the annulus there are three possible paths for fluid movement; the interfaces between cement/rock and cement/casing and the cement matrix.
Poor mud removal is normally identified as the major source of communication problems, although poor bonding at the interface can occur even when mud cake or oil films have been completely removed. Mud removal is not included in the scope of this work and it is assumed that state of the art techniques are used on every well to eliminate this problem.
Cement bonding can also be affected by slurry properties like fluid loss and free water. However, cement adherence to the formation and casing is primarily affected by cement shrinkage and by stress changes induced by downhole variations of pressure and temperature. These occur mainly inside the casing but can also originate in the formation.
By convention, when hydrostatics allow, two types of slurries are used to cement a casing string. A neat (or tail) slurry, with a density ranging from 15.5 lbm/gal to 16.5 lbm/gal depending on cement type and well BHST (bottom hole static temperature), is used to anchor the casing.