Drilling Through Salt: Constitutive Behavior And Drilling Strategies (2004)

Drilling through salt sections requires that the particular properties of salt, its creep behaviour and high solubility, be recognized and incorporated in the drilling plan. Salt is a viscous material and creeps under differential stress; the creep rate is a strong function of both temperature and stress difference (actually underbalance between the mud pressure and the vertical stress). A simple model approach to account for these effects in a reasonably quantitative manner is described. Problems encountered in drilling through salt include hole closure leading to stuck tools, differential dissolution of beds of carnallite, bischofite and other halides, encountering stiff and non-viscous stringers in salt strata, and exiting salt into non-salt rocks, always a challenging phase of the drilling. Strategies for successful salt drilling involve recognizing salt closure behavior, stresses, and adjusting drilling fluid density and temperature to minimize problems. Casing design issues in salt are also discussed. Figure 1 shows cases ranging from shallow bedded salt to deep diapiric structures. In salt tongues (sheets) and bedded salt cases, well trajectory choice is limited, but with diapiric structures, a wide range of drilling trajectory choices exists, including paths which avoid salt altogether, and other paths designed to pass through as much salt as possible. Which is chosen depends on the geomechanical state of the rocks surrounding the salt, the depth (T, σ) and creep rate of the salt, and other factors, some experiential, some gleaned from analysis. Assuming that salt dissolution issues in the drilling fluid are properly managed, the two major concerns are borehole closure through creep, and borehole instability when exiting salt or in cases of sharp lithology contrasts (e.g. anhydrite/salt interfaces). Casing point choices must account properly for stresses so that drilling can proceed at low blowout and lost circulation risk. To manage drilling risks, it is necessary to: • Determine the in situ conditions, T(z), σ(z) • Choose a suitable constitutive law for creep • Calculate borehole closure rates as functions of depth and activating stress (σv – pb) • Assess the risks and if necessary execute thermal (cooling and heating) calculations • Choose the best mud weight program for the casing program and trajectory • Monitor hole performance and be prepared to modify the program appropriately • Be prepared to act decisively when exiting thick salt structures. Figure 1 (Left and Below): Some Conditions Encountered in Drilling Salt 2. IN SITU CONDITIONS 2.1. Stresses and Temperatures Figure 2 shows a profile of a thick salt tongue in the GoM. The temperature is ~0°C at the sea floor, and ~80°C at 5.5 km subsea. In the cases sketched in Fig 1, the geothermal gradient is on the order of 2025°C/km, typical values for sedimentary basins, but there is a lower geothermal gradient, on the order of 15°C/km in the GoM deep offshore area. These low T gradients facilitate salt drilling, compared to North Sea cases where gradients within salt approaching 38°C/km have been found. Because salt rocks are viscous and flow slowly at all non-zero shear stress states, one may assume that σv = σHMAX = σhmin = z ⋅ γ , where γ is the mean overburden bulk density. Careful measurements in mines and LOT or FIT values obtained during salt drilling all tend to confirm this condition. Isotropic stresses are only found in viscous rocks and very Z km