Wellhead Movement Analysis and Surface Casing Integrity in Pre-Salt Wells

As a general practice in the oil and gas industry, the well foundation, composed by the conductor and the surface casing, is designed with a strict tolerance regarding cement shortfall on the surface casing. However, in a pre-salt scenario, in order to reduce the costs of well construction, the surface casing shoe generally reaches the top of salt. In this case, it is quite hard to make the cement job reach the mudline due to problems like salt dissolution (generating high calipers) and presence of many geological faults in the post-salt zone (which can work as a lost circulation area). Besides that, an evaluation of the wellhead movement is necessary so that the structural restrictions of subsea equipment connected to the wellhead are not violated. This work had the goal of presenting a coupled structural model to analyze the foundation of a subsea well with a partially cemented surface casing, where the safety factors of surface casing are evaluated in the whole well life cycle along with the wellhead movement due to the loads related to each step of this cycle. A sensitivity analysis on the top of cement (measured from the casing shoe) is made, varying it from 300 m to 800 m. The results showed wellhead movement consistent with what is observed in the field, once no axial movement has been reported. Additionally, it was highlighted that the foundation design depends on the operations during the well construction and its future purpose, production or injection, because the thermal loads associated with operations have different impacts.

Alignment of saline springs with evaporite karst structures in northeast Alberta, western Canada: analogue for cretaceous hypogene brine seeps to the surface

Meteoric and glacial meltwater charged groundwater, mixed with dissolved salts from Devonian sources at depth, discharged as saline springs along topographic lows of the Athabasca River Valley, which downcuts into the Cretaceous Athabasca oil sands deposit in northeast Alberta, western Canada. These Quaternary saline seeps have TDS measurements, isotope signatures and other chemical characteristics indicative of the groundwater flows coming in contact with Prairie Evaporite (M. Devonian) salt beds, 200 m below the surface. Migrations up-section of groundwater with dissolved chloride and sulphate salts occurred along salt dissolution collapse breccia zones that cross-cut Upper Devonian limestone strata. Seeps discharged along the karstic Devonian limestone paleotopography, the unconformity surface flooring the Lower Cretaceous McMurray Formation. Saline to brine springs along the Athabasca River Valley have TDS measurements that can exceed 100,000 mg/L. Quaternary salt removal was insignificant compared to the voluminous removal of the 80-130 m thick salt section for 1000s km2 during the Early Cretaceous configuration of the Devonian paleotopography, which partially controlled depositional patterns of the overlying McMurray Formation, principal host rock of the Athabasca oil sands. Little is known of the storage or disposition of voluminous brines that would have resulted from this regional-scale removal of the salt beds below the Athabasca deposit during the Cordilleran configuration of the foreland Alberta Basin. Holocene dissolution trends and discharges at the surface as saline springs are proposed as a modern analogue for voluminous Early Cretaceous brine seeps to the surface along salt dissolution collapse breccia zones, concurrent with deposition of the McMurray Formation. This model links several characteristics of the McMurray Formation as responses to Aptian brine seeps to the surface. These include: (1) the emplacement of a drainage-line silcrete along the margins of the Assiniboia PaleoValley, now partially exhumed by the Athabasca River Valley, (2) distribution of brackish-water burrowing organisms, and (3) diagenesis of calcite-cemented sand intervals.

Kinetic Regularities and Mathematical Modelling of Potassium Chloride Dissolution

The dissolution process of potassium chloride particles in the apparatus with two-blade mechanical stirrer was investigated and the mass transfer coefficient was determined. The experimental results were generalized by criterion dependence. The independence of the mass transfer coefficient from the solid particles diameter was confirmed. A countercurrent process of potassium salt dissolution in two apparatuses with a mechanical stirring was considered. A mathematical model for countercurrent dissolution was developed and the efficiency of this process was determined.

Relating thermohaline simulation of salt dissolution to land collapse at a Transylvanian salt diapir, Romania

<p>The presented study estimates salt dissolution caused by groundwater around a salt diapir in the Transylvanian Basin, which is facing land-collapse hazards related to historic salt mining activities. Because the amount of salt dissolution is controlled by the concentration gradients and fluxes near vulnerable areas of the salt dome, specific attention has been given to hydrogeological boundary conditions. They include the hydraulic role of possible more permeable fault zones along the salt dome, and the potential access to the salt diapir of over-pressurized subsaturated groundwater within regional scale sandstone layers. A structural three-dimensional (3D) model of the salt diapir, the adjacent basin sediments, and the mining galleries was developed based on existing maps, borehole data, own field observations, and geological publications of the Transylvanian Basin. The salt dissolution potential was simulated with 2D vertical thermohaline flow and transport model scenarios along the southeastern flank of the diapir. Results showed that the following factors increase the salt dissolution capacity along the upper 180 m of the diapir: (1) the presence of more permeable Quaternary alluvial sediments in connection with a fault zone of higher permeability along the diapir, and (2) the presence of more permeable sandstone units within the Miocene sediments in the east of the diapir, which provide freshwater access to the upper parts of the diapir. Thermohaline simulation with viscosity variation of the fluid, instead of a constant viscosity, influences the resulting salt fluxes by up to 50% within studied temperature ranges of 10 to 60°C in the model domain. The range of theoretical dissolution rates along the upper 180 m of the diapir supports the hypothesis that cavern collapse is more likely to occur where cavern side walls have already been mined to almost no remaining side walls of rock salt, which is the case in the southeastern part of the diapir. A past land collapse from 2010, which formed a 70-90 m wide saline lake, has occurred in this area southeast of the diapir appearing to be the more vulnerable to land collapse.</p><p>Zechner, E., Dresmann, H., Mocuţa, M., Danchiv, A., Huggenberger, P., Scheidler, S., Wiesmeier, S., Popa, I., Zlibut, A. (2019): Salt dissolution potential estimated from two-dimensional vertical thermohaline flow and transport modeling along a Transylvanian salt diapir, Romania, Hydrogeol. J., 27, 1245-1256, https://doi.org/10.1007/s10040-018-1912-1.</p>