The application of salt for primary cementing in the past has been restricted largely to salt formations. Recognition of its value in cementing through fresh-water-sensitive shales and bentonitic sands has recently brought about wide usage. Formations of this latter type from different areas have been sampled and tested for the applicability of salt cement with emphasis s on improved cement-formation bonding and minimization of formation deterioration by water contact. Field surveys indicate this economical additive has helped to reduce remedial work and to greatly improve the success of primary and squeeze cementing jobs. A study has also been made of the effects of various concentrations of salt in cement systems and how these concentrations modify their slurry properties.
The application and use of sodium chloride in oilwell cementing dates back over a decade. The initial recorded use of salt with cement appeared in the completion of wells through salt domes along the Gulf Coast in the 1940's. In the absence of bulk blending facilities, salt was added to the mixing water prior to mixing with cement. This practice was followed to help provide better bonding to salt formations, as illustrated in Fig. 1. Here it can be seen that the fresh-water slurry has dissolved a portion of the salt, resulting in no bonding between the two, while the salt-saturated slurry causes no solution problem and permits contact and bonding of cement and salt. The addition of sufficient sodium chloride to provide a saturated solution for mixing cement required considerable time and expense to the operator. Foaming, which was encountered during the mixing of salt water, necessitated development of anti-foam agents and it became fairly common to add 1 pt of tributylphosphate/10 bbl of salt water to minimize this nuisance. These operational difficulties and misunderstanding of the effect of salt in cement systems probably account for the long delay in widespread use of such slurries. Another application of brines for mixing cement occurred when early cementers found that certain shaly formations could be more effectively squeezed when using water from the producing zones. However, the addition of salt to the mixing water for this specific application was rarely considered, and only scattered uses are recorded. Perhaps the earliest significant use of salt cement appeared in the Williston basin area of North Dakota and Montana. The problem of collapsed casing and tubing in salt sections and investigation of the reasons for this made it a logical consideration. Here the application was to provide good bonding to salt sections. Previous developments in blending equipment made the dry blending of salt with the cement practical for the first time. Tests revealed that granulated salt added to the dry cement in sufficient quantity to saturate the mixing fluid was a practical approach to overcome previously objectionable features. Wellhead sampling showed that the mixing provided by pumping equipment resulted in solubilization of the salt before entering the wellhead. Today, practically all salt used in oilwell cementing is dry-blended with cement before delivery to the wellsite. In studying troublesome and often expensive squeeze jobs in shaly zones, salt cement was again given consideration. Success with squeeze cementing in shaly sections in Southern Oklahoma might be considered the initial application of salt slurries for shales and bentonitic sands.
The use of salt has many unique properties for oilwell cementing. Ludwig described the general effects of salt on cement and the basic chemistry involved when cement reacts with sodium chloride in concentrations ranging up to saturation of the mixing water. More recently, Beach recognized the benefits of small quantities of salt in gel cements. Salt produces two opposite effects on the setting of cement, depending on the concentration.