Salt Creep Research Articles

Rheology of rock salt for salt tectonics modeling

Numerical modeling of salt tectonics is a rapidly evolving field; however, the constitutive equations to model long-term rock salt rheology in nature still remain controversial. Firstly, we built a database about the strain rate versus the differential stress through collecting the data from salt creep experiments at a range of temperatures (20–200 °C) in laboratories. The aim is to collect data about salt deformation in nature, and the flow properties can be extracted from the data in laboratory experiments. Moreover, as an important preparation for salt tectonics modeling, a numerical model based on creep experiments of rock salt was developed in order to verify the specific model using the Abaqus package. Finally, under the condition of low differential stresses, the deformation mechanism would be extrapolated and discussed according to microstructure research. Since the studies of salt deformation in nature are the reliable extrapolation of laboratory data, we simplified the rock salt rheology to dislocation creep corresponding to power law creep (n = 5) with the appropriate material parameters in the salt tectonic modeling.

Impact of rock salt creep law choice on subsidence calculations for hydrocarbon reservoirs overlain by evaporite caprocks

AbstractAccurate forward modeling of surface subsidence above producing hydrocarbons reservoirs requires an understanding of the mechanisms determining how ground deformation and subsidence evolve. Here we focus entirely on rock salt, which overlies a large number of reservoirs worldwide, and specifically on the role of creep of rock salt caprocks in response to production‐induced differential stresses. We start by discussing available rock salt creep flow laws. We then present the subsidence evolution above an axisymmetric finite element representation of a generic reservoir that extends over a few kilometers and explore the effects of rock salt flow law choice on the subsidence response. We find that if rock salt creep is linear, as appropriate for steady state flow by pressure solution, the subsidence response to any pressure reduction history contains two distinct components, one that leads to the subsidence bowl becoming narrower and deeper and one that leads to subsidence rebound and becomes dominant at later stages. This subsidence rebound becomes inhibited if rock salt deforms purely through steady state power law creep at low stresses. We also show that an approximate representation of transient creep leads to relatively small differences in subsidence predictions. Most importantly, the results confirm that rock salt flow must be modeled accurately if good subsidence predictions are required. However, in practice, large uncertainties exist in the creep behavior of rock salt, especially at low stresses. These are a consequence of the spatial variability of rock salt physical properties, which is practically impossible to constrain. A conclusion therefore is that modelers can only resort to calculating bounds for the subsidence evolution above producing rock salt‐capped reservoirs.

Modelling of a deepwater Brazilian field to assess fault reactivation and the insitu stresses during production

Fault reactivation and stress distribution are considered during a 30-year production period on a deep-water oil field using a 3D finite element geomechanical numerical simulation. Field geometry is constructed as a geomechanical model with the simulation carried out in terms of total stress; the model incorporated a network of 54 pre-existing faults which permits actual slip to occur. The principal objective is to determine whether the existing faults would propagate through to the seabed during production. The pre-production phase aims to capture material and stress states of the field accounting for salt creep and stress realignment due to the presence of the predefined faults. Preproduction stresses show good agreement with minifrac experimental results; the 3D simulation overestimates the minimum horizontal stress by 1.7-3.7%. The production stage shows that vertical compaction of the reservoir is the major influence on fault slip; the rate and magnitude of the fault slip is directly related to the rate and magnitude of depletion. The maximum fault slip is 0.82 m with normal faulting occurring beneath and thrust faulting occurring above the reservoir. Fault slip is not significant close to the seabed and the pre-existing faults are not anticipated to propagate to the seabed.

Design Procedure for Cementing Intercalated Salt Zones

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper OTC 26310, “Design Procedure for Cementing Intercalated Salt Zones,” by S.R.K. Jandhyala, SPE, and K. Ravi, SPE, Halliburton, and J. Anjos, Petrobras, prepared for the 2015 Offshore Technology Conference Brasil, Rio de Janeiro, 27–29 October. The paper has not been peer reviewed. Wells often require being drilled through and cemented across salt formations. In many parts of the world, salt sections consist of multiple salt types. This analysis shows that intercalated salts subject cement sheaths to a series of tensile and compressive loads whose magnitude depends on the size and relative position of different salts. The salt/salt-interface effects dominate the general tenet of increasing creep rate with increasing depth. This work demonstrates cement design that includes evaluating cement-sheath mechanical integrity in intercalated salts. Introduction Drilling and cementing challenges associated with salt formations are well-known. One of the more significant of these is the plastic deformation of salt attributed to the existence of deviatoric (shear) stress. This deformation is known as creep. To determine the role of creep in the mechanical integrity of a cement sheath, it is necessary to analyze the thermostructural model of the well-construction process using the creep constitutive relationship. The outcome of the analysis is the stresses experienced by the cement sheath. It is possible to quantify the risk posed by salt creep and other operational loads to the cement-sheath integrity by comparing these stresses to the failure properties of the cement. To provide an accurate quantification of the risk, it is necessary to verify the creep constitutive relationship with data from experimental creep tests. The type of failure of cement sheath depends on the type of loads exerted on it. For example, compression loads are likely to result in shear failure. Similarly, extensional loads are likely to result in tensile failure. In a typical well- construction process, the in-situ stresses attributed to overburden are greater than the hydrostatic pressure offered by wellbore fluids, such as drilling mud, cement, and completion fluid. Hence, the deviatoric stress on salt formations will lead to wellbore closure. The cement sheath thus will experience compression loads caused by creeping salts. However, the compression load is not uniform along the axial direction. This is because the creep rate of salt varies with depth. The variation is severe when multiple salts with different creep rates are intercalated. Fig. 1 shows an example analysis of openhole closing in the presence of intercalated salts. In such scenarios, the salt/salt- intercalation junctions exert extensional loads on the cement sheath. The closure is not to scale, and the displacements are magnified for a better view. Thus, in the presence of intercalated salt formations, the possibility of tensile failure of a cement sheath should be investigated in addition to shear failure. Evaluating for both shear and tensile failure requires analyzing a longitudinal wellbore model.

FEASIBILITY STUDY OF UNDERGROUND SALT CAVERNS IN WESTERN NEWFOUNDLAND: EXPERIMENTAL AND FINITE ELEMENT INVESTIGATION OF CREEP-INDUCED DAMAGE

Underground caverns in rock salt deposits are the most secure disposal method and a type of gas-storing facility. Gas storage plays a vital role in ensuring that a strategic relationship is secured between an established energy infrastructure provider and a midstream energy company. The Fischells Brook area is a pillow-shaped body of salts located in the St. George's Bay area of southwest Newfoundland, which has three layers of salt beds, and is capable of excavating caverns for the storage purposes. The development of cavern facilities requires the stability analysis through numerical models and experimental facilities. This work was motivated to examine the engineering feasibility of the salt cavern characteristics in this area, and to investigate its stability under creep behavior. An experimental test facility was developed to investigate the constitutive parameters governing the creep of rock salt, and the constitutive parameters were implemented into a developed finite element model to investigate the stability of the cavern over a 5-year period. Also a stress-based dilatancy failure envelope was developed to interpret the results of the numerical model, and to conduct sensitivity analyses for different design scenarios. The design recommendations developed in this study will be implemented as a key part of an engineering feasibility study for underground caverns in salt deposits in western Newfoundland.

Steady-State Creep of Rock Salt: Improved Approaches for Lab Determination and Modelling

Actual problems in geotechnical design, e.g., of underground openings for radioactive waste repositories or high-pressure gas storages, require sophisticated constitutive models and consistent parameters for rock salt that facilitate reliable prognosis of stress-dependent deformation and associated damage. Predictions have to comprise the active mining phase with open excavations as well as the long-term development of the backfilled mine or repository. While convergence-induced damage occurs mostly in the vicinity of openings, the long-term behaviour of the backfilled system is dominated by the damage-free steady-state creep. However, because in experiments the time necessary to reach truly stationary creep rates can range from few days to years, depending mainly on temperature and stress, an innovative but simple creep testing approach is suggested to obtain more reliable results: A series of multi-step tests with loading and unloading cycles allows a more reliable estimate of stationary creep rate in a reasonable time. For modelling, we use the advanced strain-hardening approach of Gunther–Salzer, which comprehensively describes all relevant deformation properties of rock salt such as creep and damage-induced rock failure within the scope of an unified creep ansatz. The capability of the combination of improved creep testing procedures and accompanied modelling is demonstrated by recalculating multi-step creep tests at different loading and temperature conditions. Thus reliable extrapolations relevant to in-situ creep rates ( $$10^{-9}$$ to $$10^{-13}$$ s $$^{-1}$$ ) become possible.

Research on a probabilistic assessment criterion of casing deformation in an incomplete borehole in deep salt formation

Drilling and completing wells through complex salt formation is technically challenging and costing. Field data demonstrates that well casings designed by traditional safety coefficient criterion occurring failure in deep salt formation though their safety factors are greater than 1. To reveal the failure mechanism, a probabilistic computational model coupled with salt formation, defective cement and worn casing is established and analyzed using Monte Carlo simulation method. On the basis of reliability theory, the results calculated by 5000 times simulations show that the traditional safety coefficient criterion has been unable to adapt to the safety assessment of well casings under salt creep conditions. To gain a sophisticated evaluation, a new assessment criterion is established and applied to assess the security of well casings under salt creep conditions. This study provides a new perspective for revealing the failure mechanism and solutions of evaluation on well casings under salt creep conditions, which may be an alternative method to study and predict the life of well casings in deep complicated formation.

Parametric assessment of salt cavern performance using a creep model describing dilatancy and failure

The present paper discusses a systematic approach to study the time-dependent performance of underground storage caverns in rock salt. An elasto-viscoplastic constitutive model capable of describing dilatancy, short-term failure and long-term failure during creep of rock salt was implemented in a Lagrangian finite element formulation. Using the finite element formulation, a series of numerical analyses was performed to investigate the influence of the cavern geometry and non-salt interbeds on the performance of underground storage caverns excavated in rock salt in the framework of large displacements. The attained results demonstrated the capability of the proposed approach in performance assessment of the underground caverns.

Allowable pillar width for bedded rock salt caverns gas storage

Abstract Three-dimensional numerical models of bedded rock salt cavern gas storages of Jintan salt mine, Jiangsu province, China, are established using FLAC 3D simulator to investigate factors affecting the allowable width for pillars between two adjacent caverns. Vertical stress, deformation, plastic zone, safety factors, and seepage pressure of pillars between two adjacent salt caverns are discussed, and the allowable width of the pillars is optimized. Results show that the vertical stress on the pillars increases with depth and decreases with increasing pillar width, gas pressure inside the caverns, and creep time, i.e., cavern operating duration. Rock salt creep gradually smoothens the vertical stresses on the pillars, and the effect is expected to become imperceptible after five years. An increase in gas pressure difference between the caverns increases shear stresses in the pillars, which aggravates the unevenness of stress distributions and reduces the stability of the pillars. The asynchronous injection–production mode has negative effects on pillar stability and seepage pressure, particularly for narrow pillars. Interlayer permeability and pillar width are key factors affecting seepage through the pillars. The allowable pillar width for bedded salt cavern gas storage groups in Jintan salt mine, Jiangsu province, China, is proposed based on vertical stress, deformation, plastic zone, safety factors, and seepage pressure, and is recommended to be 2.0–2.5 times the maximum diameter of a cavern. This is a smaller range than the pillar width proposed by available codes, which ranges from 1.5 to 3.0 times the maximum diameter of a cavern. This decreases the uncertainty in the pillar width design caused by the conventional wide range and can increase the efficient use of the rock salt resources. The study provides fundamental data and references for designing pillars for salt cavern gas storages in other areas with similar conditions.

Time-dependent subsidence prediction model and influence factor analysis for underground gas storages in bedded salt formations

Taking into account the impurity and creep of rock salt, a new model is proposed to predict the time-dependent subsidence of the surface above underground gas storage caverns in bedded salt formations. In this model, the shape of the storage cavern is assumed to be a sphere. Formulae are developed for the volume and volume reduction of the cavern over time during both the construction and the operating periods. A probability integral method is adopted to determine the shape of the subsidence regions, and corresponding formulae are derived for the time-dependent surface subsidence, radial tilt and radial curvature. FLAC3D was used to verify the proposed prediction model. Using the proposed model, parametric analyses are carried out to study the influences of draw angle, volume adjusting coefficient, brine withdrawal rate, internal pressure, cavern depth, material properties of the rock salt, cavern diameter, and operating time on the surface subsidence. The results show that: the subsidence increases with an increase of the draw angle, volume adjusting coefficient, brine withdrawal rate, cavern diameter, material properties of the rock salt, and time, but decreases with an increase of internal pressure. Subsidence increases first and then decreases with increasing cavern depth. Draw angle, volume adjusting coefficient, cavern depth, cavern diameter, material properties of the rock salt, time and internal pressure have a large influence on the subsidence, but there is little influence from the brine withdrawal rate and salt impurity.

A Nonlinear Creep Model of Rock Salt and Its Numerical Implement in FLAC3D

Creep characteristics are integral mechanical properties of rock salt and are related to both long-term stability and security of rock salt repository. Rock salt creep properties are studied in this paper through employing combined methods of theoretical analysis and numerical simulation with a nonlinear creep model and the secondary development in FLAC3Dsoftware. A numerical simulation of multistage loading creep was developed with the model and resulting calculations were found consequently to coincide with previously tested data.

Wellbore Stability of Salt and Anhydrite Formation of Fauqi Oilfield in Iraq

Salt and anhydrite formation of Fauqi oilfield in Iraq contains salt, anhydrite and shale. Complex situations have occurred in drilling process, such as overflow and sticking. The cores of the three lithology rock are fetched and their strength and creep mechanical properties are tested. Anhydrite is with higher strength and lower creep properties, and shale and salt is with lower strength and higher creep properties. The collapse pressure and fracture pressure of the three lithology rock are calculated. The safe density window of anhydrite is the most widest and there is no risk of wellbore instability, and the safe density window of shale is the narrowest and wellbore instability easily occur. The low limit of mud density to prevent shale and salt creep are calculated by power law model. The safe drilling density window is determined and successfully applied in drilling field.

Interbed Modeling to Predict Wellbore Damage for Big Hill Strategic Petroleum Reserve

Oil leaks were found in wellbores of Caverns 105 and 109 at the Big Hill Strategic Petroleum Reserve site. According to the field observations, two instances of casing damage occurred at the depth of the interbed between the caprock bottom and salt top. A three-dimensional finite element model, which allows each cavern to be configured individually, was constructed to investigate horizontal and vertical displacements in each well as it crosses the various interbeds. The model contains interfaces between each lithology and a shear zone (fault) to examine the interbed behavior in a realistic manner. This analysis results indicate that the casings of Caverns 105 and 109 failed, respectively, from shear stress that exceeded the casing shear strength due to the horizontal movement of the salt top relative to the caprock and tensile stress due to the downward movement of the salt top from the caprock. The wellbores of Caverns 114 and 104, located at the far end of the field and near the fault, respectively, are predicted to fail by shear stress in the near future. The wellbores of inmost Caverns 107 and 108 are predicted to fail by tensile stress in the near future. The salt top subsides because the volumes of caverns in the salt dome decrease with time due to salt creep closure, while the caprock does not subside at the same rate as the salt top because the caprock is thick and stiff. This discrepancy yields deformation of the well.

Predicting the depth of viscous stress peaks in moving salt sheets: Conceptual framework and implications for drilling

Peak-stress zones and pressure anomalies around wellbores through moving salt bodies threaten well integrity. Our study provides a conceptual framework to identify the depth of peak zones of shear stress in moving salt sheets, whether autochthonous or allochthonous. We use analytical methods to determine the principal stress magnitudes and stress-trajectory orientations caused by ductile creep in crystalline salt. The four basic flow types analyzed are Couette, Poiseuille, no-slip squeezing, and free-slip squeezing flow. The analytical formulae specify velocity gradients, shear-stress profiles, and principal stress trajectories. The analytical results are compared with detailed flow visualizations in numerical and physical models of salt tectonics to test our generic conclusions. We offer guidelines to optimize drilling operations in moving salt bodies. The study suggests that stress trajectories and zones of viscous stress peaks in salt layers should be mapped routinely during well planning, in addition to the traditional analysis of elastic stresses and safe mud-pressure windows for controlling wellbore stability.

Cost-Effective Ultralarge-Diameter Polycrystalline-Diamond-Compact-Bit Drilling in Deepwater Gulf of Mexico

Summary Ultralarge-diameter polycrystalline-diamond-compact (PDC)-bit drilling is a fast-growing cost-effective solution in high-tier deepwater drilling operations in the US Gulf of Mexico (GOM) where salt is encountered in the shallow part of the wellbore. Conventional design called for roller-cone (RC) (IADC Code 111-115) drill bits on positive-displacement motors (PDMs) in these ultralarge-diameter intervals. Cost savings on drilling fluid alone, in the form of rate-of-penetration (ROP) gains through the salt interval, has the industry trending to drill these riserless sections with the use of PDC drill bits on rotary-steerable-system (RSS) drilling assemblies. New robust high-torque-capacity topdrives, stronger drillpipe (DP) connections, larger-diameter RSS tools, and improved mud programs have all largely contributed to this step change in drilling performance. In addition, evolved bit and bottomhole-assembly (BHA) design, efficient operating parameters, improved hydraulics, and vibration-prediction modeling have all aided in the success of these runs. Although this emerging new trend reduces drilling times and associated cost, experience has shown there are multiple challenges that must be overcome to complete a successful run in a single trip. These challenges vary from well to well and include, but are not limited to, BHA steerability, rig-equipment limitations, efficient operating parameters, identification of both sediment and salt formations, hole cleaning and hydraulics, salt creep, drilling-fluid displacement, DP torque limitations, stabilization placement, lateral/ torsional BHA vibrations, and others. This paper will concentrate on the multiple aspects of ultralarge-diameter riserless PDC-bit drilling applications and the considerations that have been used to optimize them. Prior SPE papers and data from previous deepwater GOM case histories were heavily researched and scrutinized to support the conclusions provided within the body of this paper. Together with industry experience available, these findings have resulted in a set of defined recommendations, providing operators with a guide to justify a lower-cost-per-foot approach through the potential reduction of drilling time in these challenging applications.

Closure of open wellbores in creeping salt sheets

Safe exploration and production of pre-salt (or subsalt) hydrocarbons require that drilling operations be optimized. We introduce analytical models of wellbore closure, accounting for variations in both the wellbore net pressure and far-field flow rate of an autochthonous or allochthonous salt sheet penetrated by the wellbore. We demonstrate how closure rates of such a wellbore evolve for increasingly stronger Rankine flow. For high viscosity salt (?10^18 Pa s) the wellbore closes by a pure sink flow without any Rankine shift from its vertical trajectory path. For low viscosity salt (?10^16 Pa s) Rankine flow dominates.Wellbore closure in salt sheets may vary within the same wellbore due to differential tectonic creep rates at different depths. Mitigation of wellbore closure by, for example, reaming, jarring, brine solution and thermal control, is most effective if spatial variation in closure rates is understood and quantified. Evaluation of wellbore closure rates due to salt creep should be customarily included in wellbore stability analyses before drilling and during well monitoring.

Numerical Simulation on Open Wellbore Shrinkage and Casing Equivalent Stress in Bedded Salt Rock Stratum

Most salt rock has interbed of mudstone in China. Owing to the enormous difference of mechanical properties between the mudstone interbed and salt rock, the stress-strain and creep behaviors of salt rock are significantly influenced by neighboring mudstone interbed. In order to identify the rules of wellbore shrinkage and casings equivalent stress in bedded salt rock stratum, three-dimensional finite difference models were established. The effects of thickness and elasticity modulus of mudstone interbed on the open wellbore shrinkage and equivalent stress of casing after cementing operation were studied, respectively. The results indicate that the shrinkage of open wellbore and equivalent stress of casings decreases with the increase of mudstone interbed thickness. The increasing of elasticity modulus will reduce the shrinkage of open wellbore and casing equivalent stress. Research results can provide the scientific basis for the design of mud density and casing strength.

Analysis of Casing Strength after Casing Wear in Ultra-Deep Rock Salt Formation

To alleviate the problems of casing collapse induced by the coupling effect of rock salt creep and casing wear, the effects of salt creep, attrition rate and casing abrasive position on the equivalent stress on casings in non-uniform in-situ stress field is analyzed by finite-difference model with worn casing, cement and salt formation. It indicates that, creep reduces the yield strength of worn casing to a certain extent; Equivalent stress on casings is bigger and more non-uniform when the abrasion is more serious; Wear position obviously changes the distribution of equivalent stress on casing, and when the wear located along the direction of the minimum in-situ stress, equivalent stress on casing could be the largest that leads to the casing being failed more easily. Equivalent stress on casings increases gradually with creep time increasing and will get to balance in one year or so; In addition, new conclusions are obtained which are different from before: the maximum equivalent stress on casings is in the direction of the minimum horizontal stress, only when the attrition rate of the casing is little; otherwise, it is not. This method could help to improve the wear prediction and design of casings.

Creep and Dilatancy Behavior of Rock Salt around Underground Caverns

In the present paper, the time-dependent behavior of a salt cavern was analyzed using finite element method. A viscoplastic model considering inelastic volume changes was utilized to describe the time-dependent mechanical behavior of rock salt. The model was then implemented in a finite element procedure to analyze the stress and deformation of rock salt around an underground cavern using an axisymmetric representation. Finally, finite element analyses results were compared with those obtained by a viscoplastic model neglecting inelastic volume changes.

Modeling time-dependent behavior of gas caverns in rock salt considering creep, dilatancy and failure

In this paper, a numerical model of the time-dependent behavior of underground caverns excavated in rock salt is presented. An elasto-viscoplastic constitutive model is utilized to describe dilatancy, short-term failure as well as long-term failure during transient and steady state creep of rock salt. The constitutive model is employed by a Lagrangian finite element formulation to simulate the stress variation and ground movement during creep of rock salt around the cavern within the framework of large deformations. Finally, finite element analyses are provided in order to investigate the efficiency and applicability of the proposed computational model to represent time-dependent response of gas storage caverns in rock salt.