3D Geomechanical Modeling of Salt-Creep Behavior on Wellbore Casing for Presalt Reservoirs HanYi Wang, The University of Texas at Austin; and Robello Samuel, Halliburton Summary Exploration drilling is venturing into deeper regions of water. During the exploration of these deeper water depths, large hydro- carbon deposits have been found below salt formations. These reservoirs are called “presalts,” which are below the salt forma- tions. Presalt reservoirs have been found in offshore Brazil, the Gulf of Mexico (GOM), West Africa, and the North Sea. Comple- tions in salt formations can be difficult because of the creep behavior that the salt formations exhibit. Creep behavior results from the instability of the salt formation, which causes a slow flow and permanent deformations. Creep deformation occurs over time and begins once the salt formation has been penetrated. Completion of the wellbore does not stop formation creep. The constant creep of the salt formation causes excess stress on the wellbore casing, which may eventually cause the casing to col- lapse. In this study, a 3D geomechanical model is developed, by use of data such as wellbore pressure and temperature; formation stress and temperature; rock, cement, and casing properties, to predict the effects of salt-creep behavior on stress loading in the wellbore casing, which helps to assess the life expectancy of wells in presalt reservoirs. The simulation results of this model can pro- vide quantitative results of casing stress and deformation as a function of time under various temperature, in-situ-stress, and operation conditions, which can be used as useful information for subsequent wellbore-casing design and wellbore-integrity analy- sis. In addition, possible methods that can mitigate the severity of salt mobility and reduce the risks of casing collapse are discussed. Introduction Salt is one of the most-effective agents in nature for trapping oil and gas. As a ductile material, it can move and deform surround- ing sediments, creating traps, and is also impermeable to hydro- carbons and acts as a seal. Most of the offshore hydrocarbons in North America are trapped in salt-related structures (Farmer et al. 1996), as are significant amounts in other continents around the world. Many reservoirs in the North Sea are below salt, as are large fields in the Gulf of Suez (Western and Ball 1992). In addi- tion, because of the impermeability of salt rocks, cavities opened in these rocks can act as a strategic hydrocarbon reserve (da Costa et al. 2012) to store large quantities of hydrocarbons, such as the Strategic Petroleum Reserve Project (Sobolik and Lord 2014). Salt can creep and deform. This ability is one of the unique and problematic characteristics of salt. If the overlying sediments offer little resistance, as is sometimes the case in the GOM, the salt rises, creating characteristic domes, pillows, and wedges that truncate upturned sedimentary layers. If the overburden does resist, salt can still push through, creating faults in the process. If tectonic conditions are right, extensional faulting in the rigid over- burden can open the way for salt ascent. Therefore, in the geologi- cal time frame, salt can move extensively within the subsurface and create diapirs and walls. Although these structures generate significant traps for hydrocarbons, they also present a number of drilling and completion problems. The rock salts behave in a vis- coplastic manner, which will deform under pressure. This defor- mation is called creep and occurs over time once the salt has been disturbed. Creep begins the instant that drilling penetrates the salt formation and occurs as a result of instability and formation stresses. When drilling in salt sections, openhole instability and the accompanying problems can often arise, including borehole walls weakened by incompatible drilling fluids, restrictions, and undergauge hole caused by salt creep, or hole enlargement because of dissolution (Fig. 1). Completion of the well does not stop the creep process; during the life of a well, salt movement can displace wellbore tubular, possibly causing casing failure or restricted access to hydrocarbon flow (Fig. 1). In addition, a salt intrusion can locally distort the stress field, making wellbore sta- bility impossible to predict by use of conventional geomechanical models because casing across salt zones is subjected to tension, compression, and hydrostatic loads combined with nonuniform forces, which must be included in design calculations to ensure the lifetime well integrity. In recent years, with the development of new technology, the oil and gas industry is exploring into deeper waters. In areas such as Brazil, the GOM, West Africa, and the North Sea, a tremen- dous amount of hydrocarbon reservoirs were discovered under large, thick salt formations (Greenhalgh et al. 2012; Chitale et al. 2014), which are called presalt reservoirs. These presalt reservoirs differ significantly from the subsalt reservoirs found previously (Dribus et al. 2008). Presalt wells are drilled into formations that were deposited before the emplacement of a layer of autochtho- nous salt, which is salt that remains at its original stratigraphic level. This autochthonous salt lies above older rocks and is, in turn, overlain by younger strata. By contrast, subsalt wells are drilled into formations lying beneath mobile canopies of alloch- thonous salt, which are masses of salt, fed by the original autoch- thonous layer, that rise through overlying layers and then spread laterally (Fig. 2). To reach the presalt carbonate reservoirs (Gar- land et al. 2012; Thompson et al. 2015), the well has to drill through the whole section of salt. In such cases, unlike drilling around a salt dome for subsalt reservoirs, it is not possible to design a well path that can circumvent the thick salt formation and reach the target reservoir below it (Dusseault et al. 2004). In many cases, the flow and deformation of a salt formation are slow and the associated wellbore-closure process is also not rapid enough to cause severe operational troubles. However, a poorly designed casing and cement job can increase the risk of collapse during production later on. Severe problems with casing failures in salt formations have been well-documented in the liter- ature (Cheatham and McEver 1964; Pattillo and Rankin 1981; Goodwin 1984; Rike et al. 1986). Drilling and cementing opera- tions in salt formations require careful planning to avoid unde- sired events, especially in deepwater scenarios where cost, safety, and environment issues are critical. Any failure and collapse of the wellbore can lead to a tragic catastrophe, especially in an off- shore environment. In many reservoirs, pressure depletion after long-term production can lead to reservoir compaction, movement of overburden, and subsidence of the surface above the reservoir. A compacting formation pulls the cemented casing along with it, compressing the axial dimension of the casing. Above the reser- voir, the overlying material typically elongates, and the casing Copyright V C 2016 Society of Petroleum Engineers This paper (SPE 166144) was accepted for presentation at the SPE Annual Technical Conference and Exhibition, New Orleans, 30 September–2 October 2013, and revised for publication. Original manuscript received for review 31 January 2015. Revised manuscript received for review 2 August 2016. Paper peer approved 9 August 2016. December 2016 SPE Drilling & Completion 261 Citation Export: Wang, H. and Samuel, R. 2016. 3D Geomechanical Modeling of Salt Creep Behavoir on Wellbore Casing for Presalt Reservoirs. SPE Drilling & Completion 31(04):261-272. http://dx.doi.org/10.2118/166144-PA