Recent Advances in Supercritical CO2 Fracturing: New Theory, New Technology, and Application

The shale oil reservoir in Jimusaer has the characteristics of low porosity and low permeability, resulting in significant resistance in oil flow compared with conventional oil reservoirs. Fracturing is needed to increase shale oil production. Supercritical CO2 (SC-CO2) is an ideal choice for fracturing fluid due to its unique physical and chemical properties. SC-CO2 fracturing is able to make CO2 flow into microfractures and greatly reduce the pumping pressure. New progress has been made in the application of the supercritical CO2 fracturing technology in Jimusaer. A phase control model of SC-CO2 fracturing as a function of temperature and pressure is established, which takes into account the SC-CO2 features, intrinsic energy, flow behavior in fracture and fluid filtration. In this paper, the influences of injection pressure and temperature, injection rate, temperature-pressure field, temperature gradient, and phase behavior are analyzed extensively, in addition, the phase control model and its chart of fracture are presented. The proppant accumulation height reduces by a small amount with the increase of the fracturing fluid injection rate. It is necessary to improve the proppant pumping technology by the sand embankment section and proppant concentration. The liquid transforms into supercritical fluid, when flowing in wellbores and fractures. Different fractures have different phase points, and a lower injection temperature is affected by higher injection rate, lower temperature gradient and closer position from transformation point to the end of fracture. Therefore, in order to achieve a better fracturing effect, the injection temperature, pressure, and rate need to be optimized by surface equipment according to the reservoir conditions, to control the phase behavior of CO2. We built a phase control model for the SC-CO2 fracturing technology, which considers temperature control. We also developed some new techniques to improve SC-CO2 fracturing which is critically needed in the Jimusaer oilfield.

An Analytical Model for Predicting Temperature Fields during Fluid Circulation in a Deep-Sea Well

Summary Accurate prediction of temperatures along a well during deep-sea drilling (DSD) is significant for wellbore stability analysis. In this paper, an analytical model is developed to study the thermal behavior around wellbore during DSD. The analytical solutions for temperatures in the tubing, annulus, and formation are obtained in Laplace space, and their values in time domains are obtained by the numerical Stehfest method. A sensitivity study of temperature distribution under different injection temperature and rate, seawater depth, and wellbore length is carried out, and a comparison is made for the thermal behavior between onshore drilling and DSD. It is found that injection rate plays a dominate role in the bottomhole temperature (BHT), which decreases by more than 40°C after 6 months when it varies from 2 to 20 kg/s. Injection temperature only affects the temperature along wellbore at a depth less than 2000 m. There is large difference in the temperatures along the wellbore between DSD and onshore drilling. The difference in the temperature at the depth of seabed and bottomhole between the two cases reaches 80 and 70°C, respectively, after 1 day. In addition, the analytical model can work as a benchmark for other models predicting the thermal behaviors during DSD.

Thermomechanical Behaviour and Interface of Overmoulded Soft Thermoplastic Vulcanizate Elastomers

The influence of melt injection temperature on the thermomechanical behaviour of soft–soft overmoulded vulcanized thermoplastic elastomers (TPV) with different elastic properties was studied. Samples with two different overmoulding temperatures were tested under uniaxial loading conditions. The full deformation and temperature fields in each TPV were determined using digital image correlation technique and infrared thermography, respectively. The maximum interface strength was found to be equal to 70N for a maximum injection temperature of 260∘C, which is consistent with the fact that high temperatures promote interdiffusion between the molten TPV and the TPV insert. The two TPV have different stiffness, leading to a significant change of the interface position along the specimens during stretching and to a significant necking in the softer material. The zone of influence of the interface in terms of stretch gradient is very different in size from one TPV to the other. In addition, thermal investigations have shown that the elasticity of the two TPV is due to both entropic and non-entropic effects, the former being the most significant at large strains.

Scale effects on hybrid rocket engine performance

In the current study, the effects of scaling up a hybrid rocket engine (HRE) in size has on its performance is investigated. A HRE design from a past RU study is selected as the base model to be progressively increased in size while geometric scale is maintained, up to ten times the original’s size. A computer program employing a quasi-steady convective heat feedback burn rate model is used to conduct simulated engine firings. One finding from this study is that the drop- off in performance for this engine, in going up in size, is not as much as expected. This can be attributed to a conservative oxidizer injection temperature setting in the model, and an oxidizer-fuel ratio mixture influence for this engine that is more impactful. The results presented here however do, to some degree, concur with established trends, with respect to thrust prediction, as the reference HRE is scaled up in size.

Scale effects on hybrid rocket engine performance

In the current study, the effects of scaling up a hybrid rocket engine (HRE) in size has on its performance is investigated. A HRE design from a past RU study is selected as the base model to be progressively increased in size while geometric scale is maintained, up to ten times the original’s size. A computer program employing a quasi-steady convective heat feedback burn rate model is used to conduct simulated engine firings. One finding from this study is that the drop- off in performance for this engine, in going up in size, is not as much as expected. This can be attributed to a conservative oxidizer injection temperature setting in the model, and an oxidizer-fuel ratio mixture influence for this engine that is more impactful. The results presented here however do, to some degree, concur with established trends, with respect to thrust prediction, as the reference HRE is scaled up in size.

Evaluating the Impact of Reservoir Cooling on the Surfactant Flood Efficiency

EOR surfactants are usually formulated at the initial reservoir temperature. Is this a correct approach? Field data from three Single-Well Chemical Tracer pilots in North Africa are used to answer this question. The objectives are, first, to provide a realistic image of the temperature variations inside the water-flooded reservoir; second, to demonstrate the impact of such temperature variations on the surfactant performances; and last, to introduce a new methodology for estimating the target temperature window for surfactant formulations. During pre-SWCTT pilot tests, water injection, shut-in and back-production were performed. The bottom-hole temperature was monitored to evaluate the reservoir temperature changes (initially at 120°C) and to calibrate a thermal model. The thermal parameters were applied to the reservoir model to simulate 30 years of water injection (with its surface temperature varying between 20°C and 60°C) and to obtain a full picture of the temperature variations inside the reservoir. Multi-well surfactant injection was modelled assuming that the surfactant is only efficient within ±10°C around the design temperature. The impact of this assumption on the additional oil recovery was analyzed for several scenarios. The rock thermal transmissivity was found to be the key parameter for properly reproducing the observed data gathered in the North African pre-SWCTT tests. The measured temperature during the back-production phase demonstrated the accuracy of the thermal model parametrization. It proved that the heat exchange between the reservoir and the injected fluid is considerably less than what industry expects: the injected water temperature inside the reservoir remains far below the initial reservoir temperature even after 11 days of shut-in. When simulating various historical bottom-hole injection temperatures and pre-flush durations, the thermal model showed an average cooling radius of 275m, larger than the industry recommended well-spacing for the EOR 5-spot patterns. This was mainly due to the significant temperature difference between the historical injected water and the initial reservoir temperature. Several simulations were performed for 3 representative bottom-hole injection temperatures of 20°C, 40°C and 60°C, varying the surfactant design temperature range between the injection temperature and the initial reservoir temperature. The results showed that regardless of the injection temperature, the simulated additional oil recovery is highest when the design temperature range is close to the injection bottom-hole temperature. This is an important subject since in the EOR industry, the surfactants are usually formulated at the initial reservoir temperature and thus, the impact of the reservoir cooling on the surfactant efficiency is seldom considered. In a water flooded reservoir, the injected chemicals are unlikely to encounter the initial reservoir temperature. This results in a dramatic loss of surfactant performance especially when there is a considerable difference between the initial reservoir and the injected fluid temperatures.