Field Application of Reaming-While-Drilling Technology in the Super Deep Composite Anhydrite-Salt Layers of Kuche Mountain Front in Tarim Oilfield

The implementation of drilling technique for multiple lithology interbeds and high-pressure anhydrite-salt in the complex Mountain Front area has been completed. The plastic creep of the anhydrite-salt layers, the losses of the low-pressure sandstone, the overflow of the high-pressure salt-water, the narrow mud density window and frequent pipe-stuck occurrence are significant issues, which trigger significant engineering challenges downhole. This study presents the application of the reaming-while-drilling (RWD) technology which has led to minimize the downhole non-productive time (NPT) and achieve successful results. The RWD technique was applied in the composite anhydrite-salt formation of the Kumugeliemu group. Through optimized combination of the RWD tools, bits, reaming blades, and the mechanical analysis the drill string with shock-absorbing design and hydraulics optimization to guarantee the bit and the reamer blades have the proper pressure drop, hydraulic horsepower and flow-field distribution, the RWD was used with the vertical seeking tool drilling technology, resulting in minimum vibration and/or stick-slip, and achieving the expected rate of penetration (ROP) as well as target inclination. It improved the operation efficiency significantly while avoiding the downhole complexities at the same time. Since the geological structure of the offset well Keshen X (no RWD) is similar to Keshen XX (RWD technology was applied), a comparison between the two wells was performed. The reaming meterage in the composite anhydrite-salt layers in Keshen XX was 791 m, spending 15 days, average ROP is 3.73 m/hr. There was no overflew or loss during the drilling. It was smooth, no pipe sticking when checking the reaming effect during the wiper trip and the tripping out. On the other hand, Keshen X spent 29 days with average ROP of 1.35 m/hr to drill the 449 m composite anhydrite-salt rock. Moreover, it was difficult to trip in and trip out during the drilling, and the pipe sticking happened frequently, back-reaming frequently as well. There were losses during both the drilling and the casing running. Due to the unsmooth wellbore, this well increased additional 3 runs of reaming after drilling operation and 4 clean-out runs. 13 days later after the reaming operation, the anhydrite-salt rock creep was checked and found that the hole was still smooth, no pipe sticking existing. Hence, RWD technology has accomplished both goals of preventing the downhole complexities and speeding up drilling. The novel RWD technology can be well illustrated by presenting all the details of its application in salt-base formations.

Particle Flow Analysis of Macroscopic and Mesoscopic Failure Process of Salt Rock under High Temperature and Triaxial Stress

In order to reveal the mechanism of thermal-induced deformation and fracture development of salt rock under high temperature, the particle flow program PFC2D was used to study the triaxial compression failure process of salt rocks under different temperatures; at the same time, a combination model of Burge and Linearpbond was proposed to simulate plastic deformation and heat conduction of salt rock. Finally, the simulation results were compared with the experimental results to verify the validity of the conclusion. The simulation results show that the elastic limit points of rock gradually descend, the dilatancy points rise gradually, and the plastic deformation characteristics of salt rock become more obvious with the increase of temperature. Due to the damage of the sample, the strong chains break and disappear, increasing the proportion of the weak chains, and the high temperature intensifies the rupture of the contact between the particles in the salt rock. As the temperature increases from 50°C to 120°C, the strong chains in the rock sample decrease significantly, and the damage gradually increases; when the temperature is 150°C, the contact force decreases sharply, and the damage of salt rock is significant. According to the particle displacement cloud diagrams, it is found that the expansion direction from the middle part of the rock sample to the left and right ends is 12.08°, 9.55°, 8.2°, 6.33°, and 0°, respectively. The displacement directions of the rock sample show obvious radial expansion tendency, and the higher the temperature, the more obvious the “drum-shaped” failure phenomenon in the middle of the rock sample. During the heating process, the thermal cracks are mainly tensile cracks, and transverse cracks are gradually formed in the middle of the model. The cementation failure points at the top and bottom of the model expand in an oblique direction and form oblique cracks of about 45°. From the three different mathematical models of macroscopic and mesoscopic views, it is concluded that the effect of temperatures on salt rock is more significant after 90°C. This research is important for exploring the macroscopic and microscopic mechanics evolution of salt rock and provides a reference for determining the long-term mechanical strength of salt rock.

Numerical Analysis of the integrity of potential host rocks – modelling thermo-hydro-mechanical processes in the containment providing rock zone

The disposal of heat-generating nuclear waste in deep geological formations is an internationally accepted concept. Several repository systems are under discussion in Germany, whereby claystone, salt or crystalline rock could act as the host rock. In this contribution we focus on repository systems where the Containment Providing Rock Zone (CRZ) ensures safe enclosure of the waste and thus the geologic barrier is essential. Even though the various rock types considered differ substantially in their mechanical, hydraulic, thermal and chemical behavior, they must all meet the same safety requirements as defined by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU) in 2020. As part of these safety requirements, it must be shown that the integrity of the CRZ is guaranteed for the verification period, i.e. the retention of the properties essential for the containment capacities must be demonstrated over 1 million years. Therefore, the formation of new pathways must be avoided and temperature development must not significantly impair the barrier effect. The anticipated stresses and fluid pressures should not exceed the dilatancy strength and the fluid pressure capacity, respectively. In order to assess the compliance of these requirements, numerical modelling is an essential and powerful tool. Even though great progress has been made regarding the efficiency of computational methods, multiphysical modelling on different length scales over long time periods is still a challenging task. Moreover, since readily available solutions do not exist, adapted methods have to be developed and evaluated, in order to verify concepts and numerical implementations. The BGR gained experience in the field of thermal, hydraulic, mechanical (THM) numerical analysis of the integrity of the CRZ in salt rock and clay stone joined research projects on German disposal options. For crystalline rocks, first concepts are currently being developed within the CHRISTA II project. Compared to clay stone and salt rock, special features have to be taken into account: First of all, crystalline rock is characterized by fractures and other discontinuities. Thus, it cannot be assumed that an undisturbed area of sufficient size can be found for the entire nuclear waste. Consequently, several smaller CRZs must be defined, each providing undisturbed rock. Numerical analysis must deal with smaller CRZs and mechanical and hydraulic boundary conditions that are influenced by fractures. In addition, the processes in the individual CRZs may influence each other (e.g. Temperature distribution). Preliminary modelling approaches and results of numerical THM analyses, considering an upscaled fracture network, are presented.

Research on Construction and Operation Parameters of an Underground Oil Storage in Depleted Salt Caverns in the East of China

The crude oil price has been keeping at a low level in recent years, which made China's government put more efforts in the development of underground oil storages in depleted salt caverns. Under the initiative of "the Belt and Road", a more concrete concept which is "the Silk Road Economic Belt and the 21st-Century Maritime Silk Road" successfully connects Jiangsu Province in the east of China. Consisting of 20 depleted caverns, Huai'an project that is still under planning is one of the most successful examples that turn depleted salt caverns into underground crude oil storages in China. Each cavern takes up 24×104m3, while the project totally takes up 480×104m3. TDMA algorithm was adopted to solve the heat exchange model of oil, brine and surrounding rocks, revealing the relationship between temperature and cavern pressure. Salt rock safety factor, salt cavern shrinkage ratio, axial stress and ground subsidence were taken into consideration to establish a 3-dimension salt rock creep model for 19 depleted salt caverns, so that the caverns’ shapes were optimized. Hydrodynamics models were used to determine the oil's flow rate into and out of a 1000m deep cavern whose thermal field was simulated by software to reveal the temperature limit of oil and brine. Due to geothermal gradient and continuous heat transmission, the average temperature of oil and brine goes up from 35°C to 44.3°C within 7 years, while the inner pressure goes up from 12.96MPa to 21.93MPa in a depleted salt cavern. Salt creep ratio decreases as oil is stored in underground caverns for a longer period. Salt is hardly penetrated by oil, while the temperature change has a strong influence on caverns’ internal pressure. The thermal expansion factor and compressibility coefficient of crude oil and brine are both crucial to the temperature's effect on internal pressure. Caverns that have larger segments in their upper-middle or middle parts are more stable and resistant to salt creep than those that have larger segments in their lower parts. When oil is injected or pumped out, it is necessary to make the internal pressure lower than the static pressure of surrounding rocks. Hence, the most appropriate flow rate of crude oil is 4.5m/s. Crude oil that is stored in deep salt caverns may be heated up to 60°C due to the geothermal gradient, but the flammable gas in oil is rapidly gasified or even explodes when it is pumped out to the surface. To avoid accidents and air pollution, oil is cooled down before being delivered via pipelines. Oil tanks used to be applied by scale in China, however they are too obvious on the ground to comply with national strategic energy safety. Compared with oil tanks of similar volumes, the Huai'an underground oil storages may save the overall cost by 35.3%. It is the first time that the salt rock creep model is established in depleted salt caverns, while the conclusion overthrew the common preference of regular cylindrical caverns.

Intelligent Parameter Inversion of Fractional-Order Model Based on BP Neural Network

To explore creep parameters and creep characteristics of salt rock, an Ansys numerical model of salt rock sample was established by using fractional creep constitutive model of salt rock, and an orthogonal test scheme was designed based on uniaxial compression test of salt rock samples. A large number of training data were obtained by combining the numerical model with the experimental scheme, and the model parameters were inverted by using the BP neural network. The model parameters are used for forwarding calculation, and the results are in good agreement with the measured strain data. This shows that the model parameter inversion method proposed in this paper can obtain reasonable parameter values and then accurately predict the creep behaviour of salt rock, which provides a good technical basis for related engineering practice and scientific research in the future.