As mature hydrocarbon reservoirs become more depleted, drilling through depleted sands is becoming increasingly common. High levels of overbalance increase the risk of occurrence of lost circulation and / or differential sticking. Recently, three Gulf of Mexico wells successfully drilled through depleted sands with overbalances ranging from 3500 to 4600 psi. In each application, a glycol-enhanced water-based mud (WBM) was used. In this paper. the formulation and application of this specially formulated WBM are discussed and the results documented.
The use of glycols as additives in WBM has been increasingly common in the past several years. Work has focused on certain glycols that exhibit "cloud point" behavior. Although soluble at surface temperatures, under the correct downhole temperatures, these glycols become insoluble and form an emulsion. Dynamic filtration and lubricity data from the laboratory, coupled with field performance from the Gulf of Mexico wells indicate enhanced filtration properties. A theory for understanding the phenomenon in WBM is offered, one which exhibits many similarities to the performance of oil-based and synthetic based muds.
The problem of differential sticking and the costs associated with it have been well documented in other works. The analysis of Shivers and Domangue of 105 stuck pipe incidents between 1981 and 1990 in the Gulf of Mexico shows an average cost per incident of $627,000 and average rig time consumed to be 10.7 days. In the current climate of high day rates, the average cost of a stuck pipe incident is conceivably much higher now.
Differential sticking has been the subject of much work and can be summarized as follows: drilling through a permeable formation with an overbalance of pressure, results in loss of fluid to the formation. This loss of fluid results in the build up of a filter cake on the face of the formation, comprised of the solids in the drilling fluid. If the rate of fluid loss is too high, then the thickness of the cake increases to the point where the drill string can become imbadded in the cake and consequently stuck. Differential sticking is more likely in deviated well bores as the drill string will be in contact with the filter cake on the wellbore more than in a vertical well.
As drilling continues in mature reservoirs such as the Gulf of Mexico, drilling through depleted sands (and other depleted, permeable formations) is becoming increasingly common and high levels of overbalance are often inevitable. Higher mud densities may be required to stabilize the well bore above the depleted sands, especially in deviated wells, and to control zones of abnormally high pressure.
Another problem is the fluid density itself. Conventional fluids built with water are limited by the density of water, 8.33 lbm/gal, where as sands can be depleted to levels as low as 1 or 2 lbm/gal equivalent (Note that all-oil muds can be built with diesel at lower mud weights 7 lbm/gal and above and this approach has been successfully applied). If the mud density cannot be adjusted to the point where the overbalance is sufficiently reduced then other parameters must be optimized, primarily filtration control and lubricity. If the rate of filtration is minimized then less filter cake will build up on the inside of the wellbore, reducing the chances of contact with the drill string and therefore reducing the likelihood of differential sticking. Increasing the lubricity of the mud filter cake will result in lower torque required to free the drill string if it does come into contact with the wall cake, also reducing the chances of the pipe becoming stuck.