Qingchao Li

Simulating the effect of hydrate dissociation on wellhead stability during oil and gas development in deepwater

It is well known that methane hydrate has been identified as an alternative resource due to its massive reserves and clean property. However, hydrate dissociation during oil and gas development (OGD) process in deep water can affect the stability of subsea equipment and formation. Currently, there is a serious lack of studies over quantitative assessment on the effects of hydrate dissociation on wellhead stability. In order to solve this problem, ABAQUS finite element software was used to develop a model and to evaluate the behavior of wellhead caused by hydrate dissociation. The factors that affect the wellhead stability include dissociation range, depth of hydrate formation and mechanical properties of dissociated hydrate region. Based on these, series of simulations were carried out to determine the wellhead displacement. The results revealed that, continuous dissociation of hydrate in homogeneous and isotropic formations can causes the non-linear increment in vertical displacement of wellhead. The displacement of wellhead showed good agreement with the settlement of overlying formations under the same conditions. In addition, the shallower and thicker hydrate formation can aggravate the influence of hydrate dissociation on the wellhead stability. Further, it was observed that with the declining elastic modulus and Poisson’s ratio, the wellhead displacement increases. Hence, these findings not only confirm the effect of hydrate dissociation on the wellhead stability, but also lend support to the actions, such as cooling the drilling fluid, which can reduce the hydrate dissociation range and further make deepwater operations safer and more efficient.

Numerical analysis of wellbore instability in gas hydrate formation during deep-water drilling

Gas hydrate formation may be encountered during deep-water drilling because of the large amount and wide distribution of gas hydrates under the shallow seabed of the South China Sea. Hydrates are extremely sensitive to temperature and pressure changes, and drilling through gas hydrate formation may cause dissociation of hydrates, accompanied by changes in wellbore temperatures, pore pressures, and stress states, thereby leading to wellbore plastic yield and wellbore instability. Considering the coupling effect of seepage of drilling fluid into gas hydrate formation, heat conduction between drilling fluid and formation, hydrate dissociation, and transformation of the formation framework, this study established a multi-field coupling mathematical model of the wellbore in the hydrate formation. Furthermore, the influences of drilling fluid temperatures, densities, and soaking time on the instability of hydrate formation were calculated and analyzed. Results show that the greater the temperature difference between the drilling fluid and hydrate formation is, the faster the hydrate dissociates, the wider the plastic dissociation range is, and the greater the failure width becomes. When the temperature difference is greater than 7°C, the maximum rate of plastic deformation around the wellbore is more than 10%, which is along the direction of the minimum horizontal in-situ stress and associated with instability and damage on the surrounding rock. The hydrate dissociation is insensitive to the variation of drilling fluid density, thereby implying that the change of the density of drilling fluids has a minimal effect on the hydrate dissociation. Drilling fluids that are absorbed into the hydrate formation result in fast dissociation at the initial stage. As time elapses, the hydrate dissociation slows down, but the risk of wellbore instability is aggravated due to the prolonged submersion in drilling fluids. For the sake of the stability of the wellbore in deep-water drilling through hydrate formation, the drilling fluid with low temperatures should be given priority. The drilling process should be kept under balanced pressures, and the drilling time should be shortened.

Study on the optimization of silicone copolymer synthesis and the evaluation of its thickening performance

Silicone polymer shows high performance for thickening supercritical carbon dioxide and has become a well-known target because it is inexpensive and environmentally friendly. In this study, siloxane polymer was synthesized by a copolymerization reaction. The synthesis conditions of the silicone polymer were optimized using a Box–Behnken design, and the yield from the process was considered as an evaluation criterion in the screening of the synthesis process. The thickening effect of the polymer was evaluated using an in-house-built ball viscometer with operation pressure not exceeding 30 MPa. The experiments clearly showed that temperature is the most crucial factor for the synthesis process. At higher preparation temperatures (>90 °C), the yield significantly decreased from the process. The stability of the yield was influenced by the change in the molar ratio and amount of the catalyst used in the preparation. The most optimal preparation parameter for the synthesis was at a temperature of 90 °C, with an aminopropyltriethoxysilane-to-methyl triethoxysilane molar ratio of 2u2006:u20061, and 0.09 g of tetramethylammonium hydroxide as a catalyst. The test yield (84.51%) coordinated well with the predicted yield of 83.72%. Adding 3 wt% siloxane to pure carbon dioxide thickened it 5.7 times at 35 °C and 12 MPa. An enhanced yield trend was observed with increasing pressure and a temperature range of 35–55 °C. The application of CO2 fracturing technology can help to reduce the greenhouse effect and the environmental pollution caused by fluoropolymers as thickeners when silicone polymer is deployed as a thickener for CO2.

Modeling the Impact of Microinstrumentation in Microholes Wells and its Comparison with Conventional Casing

Microhole or slim hole drilling technique not only saves the time of drilling but also reduces the cost of drilling dramatically. The problem with this technology is its least technical acceptance due to wellbore stability of small-sized wellbores. The comparative analysis is conducted on three different wellbore models plotted against various yield strength pressures for conventional and microhole wells. Three categories of casing grades with different casing sizes were selected to develop the wellbore models, and the comparison categories include conventional wellbore (J-K-55), microhole wellbore (J-K-55), and microhole well (P-110). The result showed that magnitudes of different yield strength pressures are 90% higher in case of microhole (P-110) model. Additionally, surface axial stress is calculated to decide what type of casing is best fit for small-diameter wellbore, and the outcomes proved that higher-grade casings can yield stress 3110 psi. Further, overall comparison revealed P-110 casing grades possess higher yield strength pressure except yield strength of the pipe body; for this parameter, the phenomenon is inverse due to smaller difference of internal and external diameters. Henceforward, this paper focuses on suggesting the suitable casing grade for microholes that can yield more pressures and stay stable for longer period.

Effect of inter-cluster interference on fracture morphology in multi-cluster staged fracturing for shale reservoir

Multi-cluster staged fracturing technology is an effective measure to stimulate the reservoir properties. However, the inter-cluster interference effect is obvious when the cluster spacing is very narrow, which seriously affects the effect of fracturing. In order to understand the interference among fracturing clusters within the single fracturing section of the shale horizontal wells during multi-cluster staged fracturing, a finite element model is developed by using ABAQUS finite element simulation software. On this basis, the influences of factors on the fracture morphology are studied. The simulation results have shown that the cluster spacing is the most important factor affecting inter-cluster interference. With the increase in the distance between adjacent clusters, the interference among the fracturing fractures decreases and the propagation of different fractures become homogeneous or similar. Moreover, the increase in the elastic modulus of the shale formation promotes the propagation of the fractures longitudinally, but it hinders the crack opening of the fracture laterally. In addition, properly increasing the injection rate of fracturing fluid during fracturing is more advantageous for obtaining long and wide fractures. Besides, the effect of the fracturing fluid viscosity on fracture width is greater than that on the fracture half-length. The simulation results show the existence of inter-cluster interference comprehensively, which can provide a reference for the design and optimization of multi-cluster staged fracturing to some extent.

Synthetic process on hydroxyl-containing polydimethylsiloxane as a thickener in CO2 fracturing and thickening performance test

ABSTRACT Silicone shows an excellent performance of thickening CO2 and becomes a well-known CO2 thickener due to the low price and environmentally friendly. Siloxane copolymers could be synthesized via the copolymerization and other reactions. Optimal preparation parameters were temperature 90°C, the aminopropyltriethoxysilane-to-methyl triethoxysilane molar ratio of 2:1, the amount of catalyst 0.09 g. The yield of production coordinated well with the predicted yield of 83.72%. The structure of production is analyzed by FTIR. Adding 3% w.t. 7.2 order of magnitude could be realized at 0.3 mL·min−1 flow rate by using the silicone present at concentrations of 3 wt% at 35°C and 12 MPa. Moreover, Greenhouse effect could be improved due to the application of CO2 fracturing.

Establishment and evaluation of strength criterion for clayey silt hydrate-bearing sediments

ABSTRACT Appropriate strength criterion is the prerequisite for wellbore stability analysis. In this paper, the Modified Mohr-Coulomb criterion that considering the effects of hydrate is established based on the experimental results of the artificial clayey silt samples with hydrate. Meanwhile, the new criterion was evaluated by comparing the differences in the safe lower limit of drilling mud density calculated by both the traditional and modified Mohr-Coulomb criteria. The result demonstrates that the mud density window calculated by the new criterion is narrower, which reveals the applicability of the modified Mohr-Coulomb criterion in the analysis of wellbore stability in hydrate formation.

Development and verification of the comprehensive model for physical properties of hydrate sediment

Natural gas hydrate is widely distributed all over the world and may be a potential resource in the near future, whereas hydrate dissociation during the development affects wellbore stability and drilling safety. However, the present modeling of hydrate reservoir parameters ignored the influence of effective stress and only considered the hydrate saturation. In this paper, a series of stress sensitivity experiments for the unconsolidated sandstone were carried out, and the influence of mean effective stress on physical parameters was obtained; a comprehensive model for the physical parameters of hydrate reservoir was developed subsequently. With the help of ABAQUS finite element software, the established comprehensive model was verified by the use of the wellbore stability numerical model of hydrate reservoir. The verification results show that ignoring the effect of mean effective stress on the parameters of hydrate formation aggravates the invasion of drilling fluid into the hydrate formation. Besides, ignoring the stress sensitivity of reservoir physical parameters will underestimate the wellbore instability during hydrate drilling, which will be a threat to the safety of gas hydrate drilling. At the end of the drilling operation, the maximum plastic strain of the model for considering and not considering stress sensitivity was 0.0145 and 0.0138, respectively. Therefore, the established comprehensive model will provide a theoretical support for accurately predicting the engineering geological disasters in hydrate development process.

Phase Change Characteristics During Carbon Dioxide Flooding

Abstract This article compares several CO2 miscibility pressure predicting methods via slim tube tests, modifies the method of National Petroleum Council, and builds the method fitting for Chinas oilfield to predict the minimum miscibility pressure of CO2 flooding, which laid the foundation for the phase changing study. In addition, a set of visible pressure volume temperature device, computed tomography device, and scanning electron microscope device is induced to show the phase changing of miscible and immiscible flooding under the condition of different injection volumes and different injection rates, which may provide visualized test results to support the reliability of the critical miscibility pressure predicting method. Such a method is a revolution of tiny flow testing experiments in porous media, and has lent a way to a deeper investigation on the unique phase changing mechanism of Chinas continental oil.