Yang, Zhaopeng (PetroChina Research Institute of Petroleum Exploration&Development) | Li, Xingmin (PetroChina Research Institute of Petroleum Exploration&Development) | Chen, Heping (PetroChina Research Institute of Petroleum Exploration&Development) | Ramachandran, Hariharan (The University of Texas at Austin, Hildebrand Department of Petroleum and Geosystems Engineering) | Shen, Yang (PetroChina Research Institute of Petroleum Exploration&Development) | Yang, Heng (China National Oil and Gas Exploration and Development Corporation) | Shen, Zhijun (China National Oil and Gas Exploration and Development Corporation) | Nong, Gong (China National Oil and Gas Exploration and Development Corporation)
The block M as a foamy extra-heavy oil field in the Carabobo Area, the eastern Orinoco Belt, has been exploited by foamy oil cold production utilizing horizontal wells. The early producing area has been put into production about 10 years, existing problems of productivity declining and produced gas-oil ratio rising. Therefore, the development optimization for the early producing area should be conducted in order to obtain the more profitable oil recovery. A typical foamy oil reservoir simulation model using 5 components was created to understand the remaining oil distribution features. Based on above understandings, technical strategies were proposed for infilling well deployment in the early producing area. Results show that the gravity drainage and gravity differentiation of oil and gas during the cold production of foamy extra-heavy oil from horizontal wells by foam flooding are the main mechanisms for formation of remaining oil. And the influence factors of remaining oil distribution include horizontal well spacing, reservoir thickness, reservoir heterogeneity, interlayer distribution and reservoir rhythm. Thus tor foamy extra-heavy oil CHOP process, the enriched remaining oil area is the place between two adjacent horizontal wells with well spacing of 600m. Therefore, well infilling is an effective measure improving oil recovery factor of cold production, and the well infilling should be implemented as soon as possible to obtain better performance of cold production.
Cold heavy oil production with sand (CHOPS) is a non-thermal primary process that is widely adopted in many weakly consolidated heavy oil deposits around the world. However, only 5 to 15% of the initial oil in place is typically recovered. Several solvent-assisted schemes are proposed as follow-up strategies to increase the recovery factor in post-CHOPS operations. The development of complex, heterogeneous, high-permeability channels or wormholes during CHOPS renders the analysis and scalability of these processes challenging. One of the key issues is how to properly estimate the dynamic growth of wormholes during CHOPS. Existing growth models generally offer a simplified representation of the wormhole network, which, in many cases, is denoted as an extended wellbore. Despite it is commonly acknowledged that wormhole growth due to sand failure is likely to follow fractal statistics, there are no established workflows to incorporate geomechanical constraints into the construction of these fractal wormhole patterns.
A novel dynamic wormhole growth model is developed to generate a set of realistic fractal wormhole networks during the CHOPS operations. It offers an improvement to the Diffusion Limited Aggregation (DLA) algorithm with a sand-arch-stability criterion. The outcome is a fractal pattern that mimics a realistic wormhole growth path, with sand failure and fluidization being controlled by geomechanical constraints. The fractal pattern is updated dynamically by coupling compositional flow simulation on a locally-refined grid and a stability criterion for the sand arch: the wormhole would continue expanding following the fractal pattern, provided that the pressure gradient at the tip exceeds the limit corresponding to a sand-arch-stability criterion. Important transport mechanisms including foamy oil (non-equilibrium dissolution of gas) and sand failure are integrated.
Public field data for several CHOPS fields in Canada is used to examine the results of the dynamic wormhole growth model and flow simulations. For example, sand production history is used to estimate a practical range for the critical pressure gradient representative of the sand-arch-stability criterion. The oil and sand production histories show good agreement with the modeling results.
In many CHOPS or post-CHOPS modeling studies, constant wormhole intensity is commonly assigned uniformly throughout the entire domain; as a result, the ensuing models are unlikely to capture the complex heterogeneous distribution of wormholes encountered in realistic reservoir settings. This work, however, proposes a novel model to integrate a set of statistical fractal patterns with realistic geomechanical constraints. The entire workflow has been readily integrated with commercial reservoir simulators, enabling it to be incorporated in practical field-scale operations design.
Foamy oil flow is commonly encountered in heavy oil production from homogeneous or heterogeneous (after cold heavy oil production with sands - CHOPS) reservoirs. This can be due to a drive mechanism in the primary production (depletion of methane saturated heavy-oil) and secondary stage (gas injection after primary production). In the primary stage, among other important parameters, pressure depletion rate has been reported to be the most critical characteristic to control this type of flow. In the secondary stage, gas amount and type (sole injection of methane, carbon dioxide, propane, or combination of these) and application conditions (soaking time on cyclic solvent injection durations, depletion rate) are critical. The cornerstone of the foamy oil behavior relies on its stability, which depends on parameters such as oil viscosity, temperature, dissolved gas ratio, pressure decline rate, and dissolved gas (solvent) composition. Although the process has been investigated and analyzed for different parameters in the literature, the optimal conditions for an effective and more economical process (mainly foamy oil stability) has not been thoroughly understood, especially for the secondary recovery conditions. In this study, air has been used as an ameliorative to improve foamy oil stability. Five pressure depletion tests divided into two cases were performed. Each pressure depletion test included eight independent pressure recordings obtained from pressure transducers distributed along a sandpack holder for 48 hours. In order to reach the optimal conditions of the applications, three different pressure depletion rates were tested at 0.23 psi, 0.51 psi/min, 1.53 psi/min, and air were tested as an ameliorative for foamy oil stability. We observed that increasing pressure depletion rates increase the formation of foamy oil, however, when pressure depletion rates were too high, it may cause a negative effect in the final oil recovery factor. We also observed that injecting air into the sandpack caused an increase in the heavy oil viscosity, and the subsequent injection of methane as a solvent became more effective in generating more stable foamy oil, which resulted in obtaining a higher oil recovery factor. This novel approach is expected to improve the understanding and the use of foamy oil mechanics and to achieve a higher foamy oil stability aiming to increase the final heavy oil recovery factor.
Yang, Zhaopeng (PetroChina Research Institute of Petroleum Exploration&Development) | Li, Xingmin (PetroChina Research Institute of Petroleum Exploration&Development) | Chen, Heping (PetroChina Research Institute of Petroleum Exploration&Development) | Liu, Zhangcong (PetroChina Research Institute of Petroleum Exploration&Development) | Luo, Yanyan (PetroChina Research Institute of Petroleum Exploration&Development) | Fang, Lichun (PetroChina Research Institute of Petroleum Exploration&Development)
The foamy extra-heavy oil reservoirs in the eastern Orinoco Belt, Venezuela with high initial dissolved gas oil ratio and flow ability in situ, have been exploited by the Cold Heavy Oil Production (CHOP) method, with recovery of only 8%-12% OOIP. SAGD has proved to be one of commercially active post-CHOP processes. Whereas during the SAGD process the dissolved gas as non-condensable gas accumulated at the edges of the steam chamber causes a resistance to heat transfer between steam and oil, thus slowing down growth of the steam chamber and oil recovery. Therefore a novel SAGD process using alternate imbalance operating-pressure (AIOP-SAGD) is studied for the purpose of improving foamy oil SAGD performance.
The novel SAGD process involves multi SAGD well pairs, and with the growth of steam chambers, a significant pressure gradient is deliberately created between two steam injection wells. Moreover the higher and lower operation pressure of the two injection wells is periodically alternate. In this work, the potential evaluation and optimization of foamy oil AIOP-SAGD are studied, through extensive simulations utilizing a sector model, which is from a sector with representative oil and reservoir characteristics of Eastern Orinoco Belt, considering the mechanism of foamy oil and thermal recovery.
Simulation results indicate that the AIOP-SAGD process shows significant improvement in oil recovery, at least 10% higher than traditional SAGD. The mechanism includes two aspects: firstly the pressure gradient between two adjacent SAGD well pairs brings a sweep of dissolved gas from steam chambers; secondly, based on the flow ability of foamy extra-heavy oil, the pressure gradient helps to exploit oil between two SAGD pairs which is typically difficult to be recovered with conventional SAGD. The optimization of operating parameters shows that the optimal start time of AIOP-SAGD is when the oil rate of SAGD reaches the peak and the steam chamber extends to the top of the reservoir. High steam quality helps improve the performance of AIOP-SAGD. Moreover the parameters of alternate time, imbalance time, imbalance pressure difference were optimized.
Cyclic solvent injection (CSI)—a non-thermal process option for development of post-CHOPS (cold heavy oil production with sand) heavy oil reservoirs—works by dissolving a gaseous solvent, mainly Methane (CH4) or Carbon Dioxide (CO2), and sometimes along with Propane or Butane into the in-situ heavy oil. A foamy oil is generated in-situ and produced during the production cycle due to pressure depletion. The generation and stability of foamy oil are considered important contributing factors to increased recovery from both solution gas drive and viscosity reduction mechanisms. This study used an innovative automatic flow loop apparatus to study questions about the pressure and gas type/concentration window for foamy oil generation and stability. The flow loop was applied to accurately monitor these phenomena through a number of property measurements at continuous flow conditions. Fluids inside the apparatus were automatically flowed back and forth between two upright Isco syringe pumps controlled by a LabVIEW data aquistion and control interface. The live oil in the system was depressurized either instantaneously or at a certain pressure drawdown rate to initiate foamy oil. Then, constant-volume pressure rebound (CVPR) tests and constant-pressure volume expansion (CPVE) tests were conducted, while density, viscosity, volume, and quality of the foamy oil phase were automatically measured and logged. The test results obtained with two separate gases, CH4 or CO2, revealed several fundamental mechanisms of foamy oil behaviour. Foamy oil stability was shown to be a complex function of live oil properties, solution/injection gas type, pressure drawdown rate, and shear rate. In the CVPR tests, when crude oil was saturated with CH4 or CO2 at the same bubblepoint pressure, the same pressure depletion decrement resulted in higher rebound pressure and longer time reaching stabilization for CH4 than CO2, due to slower CH4 bubble nucleation and release from the foamy oil phase. By contrast, the CPVE tests showed that the volume and viscosity increase ratio of the foamy oil phase compared to the original live oil were strongly affected by depletion pressure and drawdown rate. This indicates that, during a CSI process, there is an optimized window of pressure depletion rate and decrement that creates stable foamy oil at non-equilibrium conditions.
Cyclic solvent injection (CSI) is a promising technology for enhanced production of heavy oil in post-CHOPS (Cold Heavy Oil Production with Sand) fields. The first stage of a CSI cycle involves the injection of solvent vapour, which re-energizes the reservoir and dissolves into oil, reducing its viscosity. The second stage of a CSI cycle involves dropping the pressure to flow the solvent-diluted oil back to the well. This is the solvent analog to cyclic steam stimulation (CSS). A key difference between solvent and steam is that the heat from steam stays in the oil even during production, while in solvent-diluted oil, if solvent comes out of solution as pressure drops, oil viscosity will increase again and one of the main benefits of the solvent will be lost. Solvent selection in CSI needs to consider which solvents have non-equilibrium properties, specifically delayed release during pressure depletion.
This study presents a set of low field nuclear magnetic resonance (NMR) tests that were run on a 3,370 mPa·s viscosity heavy oil and several solvents: methane-propane and methane-CO2. NMR measures the relaxation rate of protons in the presence of magnetic fields. When fluid viscosity decreases, the fluid signal relaxes more slowly. A properly calibrated NMR model can be used to measure the viscosity of the liquid phase in a solvent-oil mixture as pressure drops and gas leaves solution. This approach does not require flow, and can also be used to measure the
Tests were run using methane-CO2 solvents with varying CO2 concentration to study the impact of live oil viscosity on the rate of solvent release. For lower initial live oil viscosity, the solvent is able to more quickly leave solution from the oil. Additional tests run using a methane-propane solvent demonstrate the impact of varying solvent type for the same initial solution viscosity. Both CO2 and propane show significant non-equilibrium solvent release during depletion.
Measurements of non-equilibrium solvent release from oil are an important piece in the understanding and modeling of heavy oil CSI. Equilibrium PVT data shows how different solvents can affect heavy oil systems, but the non-equilibrium solvent release is crucial for these solvents to work as recovery agents in the field. The results presented in this study provide useful data for CO2 and propane as potential CSI solvents, and help in the understanding of the role of viscosity vs. solvent type in the diluted oil response.
Jiang, Youwei (Research Inst Petr Expl & Dev) | Wang, Bojun (Research Inst Petr Expl & Dev) | Li, Qiu (Research Inst Petr Expl & Dev) | Zhao, Jian (Tuha Oil Field, PetroChina) | Zhang, Yunjun (Research Inst Petr Expl & Dev) | Zheng, Haoran (Research Inst Petr Expl & Dev)
Nitrogen assisted artificial foamy oil flooding (AFO) is an oil recovery process in which nitrogen and foamy oil agent are injected into a water flooded reservoir to drive the remaining oil when it is emulsified with the injected gas and water. The rheological behavior of the emulsified oil is similar with foamy oil flow, so this process is called artificial foamy oil flooding in this paper.
A visible micro-model of etched glass is developed to observe the emulsification process of nitrogen, foamy oil agent, water and oil. The foamy oil agent is a composition of surfactants like foaming agents, foam stabilizers, viscosity reducers and so on. One-dimensional flooding experiments are conducted to measure the resistance factor of artificial foamy oil flow process. The impacts of foamy oil agent concentration and air/liquid ratio upon the resistance factor are assessed in the experiments too. The potential of AFO of overcoming the interlayer interference effect is evaluated with parallel core flooding experiments. Based on experimental results, the reservoir engineering aspects of this technique is discussed and the capability of enhancing oil recovery is demonstrated with a pilot application outcomes.
The mechanisms of AFO unveiled with the etched glass model include: (1) dramatic viscosity reduction with oil/water emulsification; (2) increased elastic energy of oil saturated with micro gas bubbles; (3) Foam blockage in water saturated zones; (4) improvement of drainage factor with pseudo miscible flooding. Compared with the simple treatment with viscosity reducer, AFO is more stable which does not separated within four hours. It reduces the oil viscosity by 80%. The resistance factor can be as high as 58 when the concentration of foamy oil agent is 0.5%. Parallel core flooding experiments reveal that the injection of nitrogen and foamy oil agent after water flooding can reduce water cut by 12%, and enhance oil recovery by 15.8%. In the pilot in Tuha Oil Field, PetroChina, the injected nitrogen compensated the formation pressure rapidly and emulsified with oil to stimulate daily oil production rate and recovery. Demonstrated with reservoir engineering theories and numerical simulation, whose outcomes are compatible with the pilot performance, artificial foamy oil flooding is of great potential to further enhance oil recovery for water flooded reservoirs in Tuha field.
Artificial foamy oil flooding technique, inspired by foamy oil flow in Venezuela, is probably a new EOR solution to reduce water cut and improve oil mobility in water flooded reservoirs, especially in heavy oil reservoirs exploited with water injection in Tuha Field. The mechanisms and recovery potential are unveiled with laboratory experiments and pilot tests.
Jin, Fu (CNPC Research Institute of Petroleum Exploration and Development & CNPC Drilling Research Institute) | Xi, Wang (CNPC Research Institute of Petroleum Exploration and Development & CNPC Drilling Research Institute) | Shunyuan, Zhang (CNPC Drilling Research Institute)
Located in south of Eastern Venezuela Basin, Orinoco Oilfield is the unique huge ultra-tight oilfield that has not been developed by scale in the world. The high-density tight oil is known for its high content of acids, heavy metals and asphaltenes with a viscosity of 1000-10000mPa·s. ML Block whose OOIP is 178*108bbl is situated in east of the oilfield, while cluster horizontal well drilling and cold production technologies are still under research there.
Based on precise geological researches numerical simulation was carried out to optimize cold production of ultra-tight oil with foamy oil flow patterns in horizontal wells, including optimization of well placement, well spacing and horizontal section length. The near-bit geo-steering drilling technology was applied on adjacent wells to test its performance, while an experiment was conducted with PVT apparatuses to examine the effect of pressure decline rates on foamy oil flow. A long core pressure depletion test was accomplished to reveal the effect of foamy oil flow on recovery factors.
Three-dimension cluster horizontal well drilling and completion technologies shall be applied to develop ultra-tight oil reservoirs in huge loose sandstones, with the near-bit geo-steering drilling technology that controls landing points and horizontal sections in real time, keeping the bit move ahead along the lower boundary of the reservoir. Therefore, recovery rates may be dramatically improved due to the gravity drainage of ultra-tight oil. The most appropriate spacing of horizontal wells (500-600m) and horizontal section length (800-1200m) were determined to achieve the maximum recovery rate. The experiment proves that the recovery rate improves as the formation permeability increases, which means the "worm hole" contributes to heavy oil extraction. Boreholes with relatively large diameters, extensive perforated holes and slotted liners may be used to complete wells. In order to take the most advantages of the foamy oil flow mechanism high displacement ESPs shall be used with the selected thinner squeezed at the bottom, otherwise PC pumps with the thinner added at the wellhead are recommended.
Cold production technologies applied in ML Block save the overall production cost by 15.2%, improving the ultimate recovery rate by 8.6%. The foamy oil flow theory is improved, while it is the first time to integrate foamy oil flow production technologies with cluster horizontal well drilling technologies and near-bit geo-steering drilling technologies. As a result, the overall production rate of tight oil was greatly improved and the average production life of wells was extended.
Primary recovery of heavy-oil is remarkably low due to high viscosity and low energy by solution gas exsolution to drive the oil. Gas injection to improve foamy flow and also to dilute the oil in such reservoirs has been proposed as a secondary recovery method. However, because of the high costs of injected gases, efforts are needed to optimize the process by selection of proper gas type (or gas combinations) and suitable injection scheme. To achieve this goal, an experimental procedure was followed with rigorous analyses of the output. A 1.5 m long and 5 cm diameter sand-pack was first saturated with brine, which was replaced with dead oil. Then, gas solvents were injected to dead-oil containing core-holder until nearly reaching 500 psi followed by a two-day soaking period. Pressures all along the sand-pack were recorded with eight pressure transducers. Different combinations of various gas solvents (methane, CO2, and air) aiming to select the most competitive and economic formula were tested with a certain set of pressure depletion rates.
The physics of the foamy oil flow for different solvent mixtures and depletion conditions were analyzed using pressure profiles acquired, recorded oil/gas data with time, and gas chromatography and SARA analyses of the produced gas and oil. Three huff-n-puff cycles were applied. Compared with other light hydrocarbon solvents and carbon dioxide, air has its high advantage in terms of accessibility and lowered cost. Hence, attention was given to air that was mainly used to pressurize the system and increase oil viscosity due to oxidation process with an expectation of better foam quality when injected with other gases such as CO2 and methane. Methane (CH4) yielded the quickest response in terms of gas drive but, in the long run, CO2 was observed to be more effective technically. Air was observed to be effective if mixed with CO2 or methane from an economics point of view. To sum up the results, air Huff-n-Puff (HnP) followed by 2-cycles of CH4 HnP yielded 36.21% recovery, while air HnP followed by 2-cycles of CO2 HnP delivered 30.36% oil. When the gases are co-injected, air 50%-CO2 50% and air 50%-CH4 50% recovered 29.85% and 23.74% of total oil-in-place, respectively.