A New Cementing Approach to Improve and Provide Long-Term Zonal Isolation

Abstract

A new cement system has been developed to meet oil and gas industry's need for better zonal isolation. In this paper, we present design, execution, and evaluation of the system used to cement solid expandable tubular (SET) in a carbonate field in southern Saudi Arabia. The SET is 900-ft long with 5½-in. diameter, placed horizontally in a 7½-in. drilled hole with a 9,000-ft depth. To enhance well production, two windows were milled from inside the cemented SET to drill laterals to maximize surface contact with the horizontal oil reservoir. Special laboratory techniques were used to evaluate set-cement and slurry properties of the system to determine its potential use for field application. Distinguishing features of the developed versus conventional cement are also explained. Pressure and inflow tests were used along with cement evaluation tools to evaluate the success of the operation. Production profile was also studied to ensure durability of the cement system. Introduction The use of horizontally drilled wells has increased dramatically during the past decade because they offer greater contact with the reservoir rocks,1Fig. 1. Smart completion technology is used widely in Saudi Arabia to aid in increasing oil production by monitoring the progress of lateral producers. The use of expandable tubing reduces the cost of the well by eliminating operations associated with conventional tubing. Saudi Aramco started utilizing SET technology in the middle of 2003 and continued field applications ever since. In this paper, we describe cementing the 5½ inches SET, that expands to a larger diameter equal to 6.147 inches. Table 1 gives some of the SET data. Long term durability of cement is an important character that needs to be considered when cementing SET applications. Milling the cemented SET could introduce problems to conventional cement. The effects of milling on conventional cement compared to the developed cement need to be investigated in the future. However, stress modeling showed that conventional cement can fail pressure and temperature cycles. 2 On the other hand, casing corrosion is another challenge. Poor cementing placement practices, set cement matrix properties and cement losses are significant factors that are contributing to the corrosion problem. Consequently, additional costs are enforced when workover rigs are mobilized to squeeze off the casing leaks or plug casing to casing annulus pressure, or squeeze off corroded casings or even run Tie-back liners. This problem can be addressed applying cathode protection or running chrome-plated tubing to protect it from being corroded. 3 Throughout the life of the well, the cement sheath is exposed to changes of the down hole conditions due to:Opening windows for multilateral wellsTemperature increase in the production processPressure increase in the well bore due to gas productionChange of the mud after drilling to a lighter completion fluidChanges of the mud weight during drilling different formationsStimulation treatmentsFormation loading (creep, compaction, faulting)Hydration of cement. All of these variations in the down hole conditions lead to stresses induced in the casing, the formation, and the cement sheath. The usual perception for judging the mechanical properties of cement is to look at the compressive strength, the higher the number the better the cement is. However, due to the above conditions, the cement failure would occur mostly in tension, debonding. Due to this expansion of set cement matrix cement elasticity, tensile strength and permeability are critically considered in order to access the effectiveness of the cement in maintaining long-term zonal isolation.