Thief Zones Research Articles

A Novel Quantitative Water Channeling Identification Method of Offshore Oil Reservoirs

Offshore oilfields are characterized by loose sandstone reservoirs, strong heterogeneity and high injection and production intensity. Water channeling gradually develops after entering the high water cut stage, which weakens production performance. Current identification methods usually have high computational costs and low efficiency. A quantitative identification model of water channeling based on inter-well connection units has been established by simplifying the complex reservoir system into a connection network between injectors and producers, which can quickly and accurately obtain strength characteristic parameters for waterflow channels. In addition, a comprehensive evaluation factor M and classification standard for water channeling suitable for offshore heterogeneous reservoirs have been proposed. It indicates a thief zone when M is larger than 0.65, a predominant waterflow channel when M is between 0.55 and 0.65, and no water channeling when M is smaller than 0.55. The application of (an) offshore S oilfield demonstrates that the new method successfully identifies 18 segments of the thief zone and 19 segments of the predominant waterflow channel and improves computational speed by 100 times compared with the conventional numerical modeling method. This novel method allows for rapid and accurate identification and prediction of water channeling, including location, directions, and strengths, thereby providing timely and practical guidance for inefficient water channel treatment.

Nanoparticle-stabilized CO2 foam flooding for enhanced heavy oil recovery: A micro-optical analysis

Surfactant flooding is a well-known chemical approach for enhancing oil recovery. Surfactant flooding has the disadvantage that it cannot withstand the harsh reservoir conditions. Improvements in oil recovery and release are made possible by the use of nanoparticles and surfactants and CO2 co-injection because they generate stable foam, reduce the interfacial tension (IFT) between water and oil, cause emulsions to spontaneously form, change the wettability of porous media, and change the characteristics of flow. In the current work, the simultaneous injection of SiO2, Al2O3 nanoparticles, anionic surfactant SDS, and CO2 in various scenarios were evaluated to determine the microscopic and macroscopic efficacy of heavy oil recovery. IFT (interfacial tension) was reduced by 44% when the nanoparticles and SDS (2000 ppm) were added, compared to a reduction of roughly 57% with SDS only. SDS-stabilized CO2 foam flooding, however, is unstable due to the adsorption of SDS in the rock surfaces as well as in heavy oil. To assess foam's potential to shift CO2 from the high permeability zone (the thief zone) into the low permeability zone, directly visualizing micromodel flooding was successfully executed (upswept oil-rich zone). Based on typical reservoir permeability fluctuations, the permeability contrast (defined as the ratio of high permeability to low permeability) for the micromodel flooding was selected. However, the results of the experiment demonstrated that by utilizing SDS and nanoparticles, minimal IFT was reached. The addition of nanoparticles to surfactant solutions, however, greatly boosted oil recovery, according to the findings of flooding studies. The ultimate oil recovery was generally improved more by the anionic surfactant (SDS) solution including nanoparticles than by the anionic surfactant (SDS) alone.

Optimizing cluster spacing in multistage hydraulically fractured shale gas wells: balancing fracture interference and stress shadow impact

Horizontal drilling and multistage hydraulic fracturing have seen widespread application in shale formations during the past decade, leading to significant economic productivity gains through the creation of extensive fracture surfaces. The determination of the ideal cluster spacing in shale gas wells is contingent upon the unique geological and formation characteristics. Generally, reducing the spacing between clusters has the potential to augment gas recovery, albeit at the expense of higher drilling and completion costs, as well as the influence of stress shadows on fracture propagation. This study introduces an integrated methodology designed to explore the impact of cluster interference on well performance. Commencing with a fracture propagation model accommodating stress shadow effects for an equivalent slurry volume injection, analytical rate transient analysis (RTA) was amalgamated with reservoir numerical simulation to compute the effective fracture surface area (Aca.) for hydrocarbon production. The correlation between the effective fracture surface area determined by RTA and the actual stimulated fracture area (Aca.) derived from numerical simulations was established in relation to cluster spacing. The findings of this research reveal that wells featuring a greater number of stages and tighter cluster spacing tend to exhibit elevated cluster interference, resulting in a lower effective-to-actual fracture surface area ratio and heightened stress shadow effects impeding fracture propagation. A cluster spacing of 33 feet with six clusters per stage emerges as the optimal choice at formation permeability of 0.00005 md that decreased to 18 ft at formation permeability of 0.00001 md. ACe either stabilizes or decreases above the optimal value, suggesting that more clusters would not have a major impact on increasing the effective stimulated area. Allowing 20% interference, regardless of the permeability of the formation, maximized cumulative production while preventing thief zones and excessive cluster interference. The insights gained from this study will serve as a valuable resource for completion and reservoir engineers, enabling them to fine-tune cluster spacing to maximize well revenue in the dynamic landscape of shale gas extraction.

APPLICATION PROGRESS OF INSITU POLYMER GEL IN OILFIELD CONFORMANCE CONTROL TREATMENT

Many oilfields around the world are using enhanced oil recovery methods to maximize oil production. Longterm water flooding processes have led to water channeling in mature reservoirs, which is a severe problem in oilfields. Polymer gel is widely used as a plugging agent to reduce water production. Previously, the conventional polymer gel is extensively used for blocking the thief zones, but the performance of conventional polymer gel is not satisfactory in high salinity and high temperature conditions due to rapid syneresis and thermal degradation. The amphiphilic polymer is taking much attention for polymer gel formulation because they are more salt resistant in low concentration compared to high concentration conventional HPAM polymer. In this paper, the crosslinking mechanism of insitu polymer gel is reviewed. The related difficulties and development prospects of polymer gels are presented. It provides a basis for the application of polymer gel in oilfield conformance control treatment. This will help researchers to develop polymer gels to improve oil recovery under economic conditions to meet the requirements of oilfields.

Interpretation of Mud Losses in Carbonates Based on Cuttings Description, Well-Logging, Seismic and Coherency Data

Hartha Formation is an overburdened horizon in the X-oilfield which generates a lot of Non-Productive Time (NPT) associated with drilling mud losses. This study has been conducted to investigate the loss events in this formation as well as to provide geological interpretations based on datasets from nine wells in this field of interest. The interpretation was based on different analyses including wireline logs, cuttings descriptions, image logs, and analog data. Seismic and coherency data were also used to formulate the geological interpretations and calibrate that with the loss events of the Hartha Fm. The results revealed that the upper part of the Hartha Fm. was identified as an interval capable of creating potential mud losses, resulting in high NPT. This is due to its diagenetic features such as succrosic dolomites and vuggy zones that could act as thief zones. Seismic potential was used for the prediction of the geological related non-productive drilling time in the Hartha interval. The seismic data quality in this interval was good, with geological observations already made. Detailed interpretation and analysis of the Hartha interval were performed and integrated with the existing seismic interpretation, rock properties, and NPT database to calibrate wells with the loss events to the seismic observations.

Impacts of Different Operation Conditions and Geological Formation Characteristics on CO2 Sequestration in Citronelle Dome, Alabama

Major concerns of carbon dioxide (CO2) sequestration in subsurface formations are knowledge of the well injectivity and gas storage capacity of the formation, the CO2 pressure and saturation plume extensions during and after injection, and the risks associated with CO2 leakage and fault reactivation. Saline reservoirs are considered as one of the target formations for CO2 sequestration through structural, residual, dissolution, and mineral trapping mechanisms. The boundary condition of the saline reservoir dictates the pressure and saturation plume extension of the injected supercritical CO2 that could expand over large distances. This can lead to sources of risk, e.g., leakage and/or fault reactivation due to presence of wells, thief zones, and geological discontinuities. Therefore, there is a critical need to develop a model that describes how risk-related performance metrics (i.e., the CO2 saturation plume size, the pressure differential plume area, and the pressure differential at specific locations) vary as a function of the size of injection, time following injection, injection operations, and geologic environment. In this study, a systematic reservoir modeling studies of anthropogenic CO2 sequestration in Citronelle dome, Alabama, was performed where all relevant scenarios and conditions to address the questions of the saturation and pressure plume size in the area of review (AoR) and post-injection site care (PISC) are considered. The objective for this study was firstly to systematically simulate CO2 sequestration, i.e., saturation dynamics, and pressure behavior over a range of operational and geological conditions and to derive conclusions about the factors influencing saturation and pressure plume size, post-injection behavior, and the risk associated with them, by developing third-generation reduced order models (ROMs) for reservoir behavior. Finally, to assess the uncertainty associated with our studies, Latin Hypercube Sampling (LHS) together with an experimental design technique, i.e., Plackett–Burman design, was used. Application of Pareto charts and respond surfaces enabled us to determine the most important parameters impacting saturation and pressure plume sizes and to quantify the auto- and cross-correlation among different parameters in both history-matched and upscaled models.

Experimental Investigation on the Performance of Polymer-Coated Clay/Polyacrylamide Hydrogel for Water Shutoff Treatment

SummaryPolymer gel system has been identified as having the potential for blocking and diverting water flow. However, the current polymer reported an inability to maintain its mechanical strength, limited penetration depth, and instability in reservoir conditions of high temperature and high pressure. A distinctive bentonite nanomer clay (PGV)/acrylamide (AM)-co-2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) preformed particle gel with poly(ethylene glycol-b-tetramethylene oxide) (PEGTMO) coating to control the swelling kinetics is formulated. The in-house formulated gel’s ability to block and divert water flow in a porous medium is studied. The formation recipe of the gel was achieved by numerous swelling tests as induced by brine solution under reservoir conditions. Through the swelling tests, the long-term thermal stability of the gel solution was demonstrated. The incorporation of PGV clay particles improves the swelling and mechanical properties of the gel. Premature swelling can be avoided with PEGTMO coating as it slows the swelling rate over a 10-minute period, which gives the advantage of controlling the swelling before reaching the intended site of action during coreflood experiments. The rheological behavior of the hydrogel features rubber-like mechanical behavior with a viscosity value of 1.17 cp, which displayed water-like characteristics. Further, significant permeability reduction of large fractures is demonstrated by the coreflooding experiment with a calculated result of 96.2%. This formulated gel could offer the solution as a blocking agent in void space channels containing reservoirs that leads to a reduction of water cut due to thief zones.

Effect of gilsonite on mechanical and thermal properties of class H cement

With an increase in global energy demands, the importance of well integrity has become more important than ever as it guarantees the continuous supply of fossil fuel to fulfill the requirement of the world. In well integrity, cement plays a crucial role as it seals/isolates the troublesome formation or thief zone meanwhile protect the casing from corrosion and giving structural support to it. Therefore, cement slurry characteristics must be according to the subsurface environment, hence properties of cement can be modified by the addition of additives. This paper focuses on the evaluation of mechanical and thermal properties of 4% gilsonite cement composite and was compared with neat Class H cement. The compressive strength of the cement is measured through a direct and indirect methods. Samples were cured at high temperatures (75 °C) and at ambient conditions for the time period of 3, 7, 14, 21, 28, and 35 days. As for the measurement of the thermal properties, samples were exposed to dry and wet conditions for a period of 60 days. It was found that at high temperature (HT) the development of compressive strength in 4% gilsonite cement composite was very rapid with the UCS going as high as 42 MPa within three days of curing. Whereas a gradual increase in the compressive strength was observed for the samples cured at room temperature (RT). For thermal conductivity (k) it was found that with the increase in the curing age the value of k decreases which showed the insulation property of gilsonite.

Three-dimensional characterization of karstic dissolution zones, fracture networks, and lithostratigraphic interfaces using GPR cubes, core logs, and petrophysics: Implications for thief zones development in carbonate reservoirs

Fractures and karstic dissolution features have been recognized to significantly affect subsurface fluid flow on a range of scales in carbonate rocks. However, due to the information gap between seismic and borehole scales, characterizing these features is a real challenge. We present a three-dimensional multidisciplinary approach to identify the main factors controlling the formation of connected rock volumes composed by permeable karstic dissolution zones. We investigate two outcrops in the Jandaíra Formation, which is an intensely fractured and karstified carbonate platform in the Potiguar Basin, Brazil. Two GPR cubes are jointly interpreted with cored boreholes, borehole images, petrophysical profiles, and thin sections. We verified that attenuated zones of GPR signal are related to secondary carbonate changes caused by fracturing and/or diagenetic alterations, leading to the development of relatively high secondary permeability. At least three types of attenuation/karstic zones might occur: fractured or broken zones affected by fracture enlargement due to dissolution, zones submitted to intense diagenetic alterations, and void spaces as a final end-member of these two processes. Fractures are preferential routes to fluid flow, thus contributing to increase dissolution processes, particularly in layers having relatively high values of primary matrix porosity and permeability. On the other hand, relevant diagenetic alterations might occur in layers that contain high concentration of stylolites. As a result of interconnecting dissolved stylolites, zones of relatively high secondary permeability can be created regardless of the primary values of matrix porosity and permeability. Geobodies of connected volumes of karstic zones often present tabular shapes and are delimited by subvertical fractures and subhorizontal lithostratigraphic interfaces. In the reservoir scale, these tabular geobodies might compose highly connected permeable volumes of altered carbonate rocks that constitute thief zones or super-K layers.

Identification method of thief zones in carbonate reservoirs based on the combination of static and dynamic data: A case study from the Cretaceous Mishrif Formation in the H oilfield, Iraq

Carbonate reservoirs usually have a strong heterogeneity. The zones with relatively high permeability will form a channel through which fluids can easily flow. These channels are called thief zones. Thief zones have notable effects on oil or gas production, for example, high oil recovery rates at the early stage of the exploitation or an early water breakthrough during the later stage of water flooding development. Therefore, it is essential to have a precise identification of thief zones in carbonate reservoirs. In this research study, a simple approach to identify thief zones based on reservoir permeability gathered from well logging is developed. The thief zones are first identified at wells based on the lower limit value of the thief-zone permeability. This value is determined based on the dynamic production data, indicating that the thief zones identified by applying this criterion can reflect the product characteristics. Then, a zonal inter-well recognition method is adopted to identify the connectivity and distributions of thief zones in the regions far away from the well. This method is applied to identify thief zones for the Cretaceous Mishrif Formation in the H oilfield, Iraq. The reliability of the identification results is tested by the well-group injection test. The distributions of thief zones in the study region are discussed. In the study region, 12 members developed thief zones, while two members (i.e., MC1-3 and MC2-2) did not develop thief zones. Specifically, there are five members having a high level of thief-zone development. They are MB1-2C, MB2-1, MB2-2, MC2-3, and MC3-2. Comparing the distribution of thief zones with that of sedimentary microfacies, it is concluded that the thief-zone development is mainly controlled by the sedimentary microfacies and tends to occur in high-energy shoals.

Review on Microemulsions for Conformance Improvement Technology: Fundamentals, Design Considerations, and Perspectives

The area of conformance improvement technology (CIT) encompasses the application of a myriad range of conventional fluids, including polymers, gels, foams, polymer-enhanced gels and foams, bacteria, and emulsions. However, these routes show operational limitations in terms of chemical degradation, thermal degradation, formation damage, susceptibility to high salinity, and segregation within pore spaces. To mitigate these problems, it is necessary to employ conformance agents that can be effectively tuned for a wide variety of reservoir conditions. Microemulsions are a promising class of fluids that exhibit thermodynamic stability, robust structure, and tunable properties. The concept of this research is to inject the optimal (correct) dosage of surfactants, which form micelles with in situ hydrocarbons to form microemulsions. Microemulsions are characterized by direct and reverse micelles, which contribute to their intermolecular interactions responsible for fluid stability and propagation under dynamic shear conditions. Design and reservoir considerations must comprise of a number of factors, namely, salinity, pH, temperature, slug concentration, and fluid activation/placement. An optimal microemulsion can be identified by understanding the flow mechanisms while accounting for mobility control, rock permeability and heterogeneity, thief zone permeabilities, and the presence of anomalies. It has been established in this review that microemulsions help plug the high permeability pore throats via a combination of the “Jamin effect” and viscosity modification. By adoption of a proper workflow design, the reservoir may be tuned via the introduction of microemulsions to suit the needs of the industry. However, considerable research is still needed to validate the design aspects and application of microemulsions for conformance improvement.

The ways of elimination of thief zones in the wells drilled by sidetrack

The paper provides the data on the properties and features of quick hardening plugging mixtures, gypsum and gypsum-cement mortars. The agents applied for changing the main technological aspects and the regulation of their properties are shown. Conditional categories on the intensity of thief zones, the recommendations on the ratio of the major components of gypsum-cement mixture for plugging in thief zones are presented as well. The methods for delivery of the mixture to the thief zones depending on the depth of the zones and kickoff in the column are reviewed.

Influence of the Injection Scheme on the Enhanced Oil Recovery Ability of Heterogeneous Phase Combination Flooding in Mature Waterflooded Reservoirs.

With the maturity of waterflooded reservoirs, owing to serious heterogeneity, the fluid will channel through the thief zone, leading to considerable remaining oil unrecovered in the upswept area. To further enhance oil recovery (EOR) after waterflooding, the heterogeneous phase combination flooding (HPCF) was composed of a polymer, branched-preformed particle gel (B-PPG), and surfactant. For the sake of improving the economic efficiency, the influence of the injection scheme on the EOR of HPCF with an equal chemical agent cost was investigated by sand-pack flooding experiments. Then, visual plate sand-pack model flooding experiments were performed to study the swept area of HPCF under different injection schemes. Results demonstrated that the total EOR of HPCF under different injection schemes ranged from 33.5 to 39.3%. Moreover, the EOR of HPCF under the alternation injection (AI) scheme was the highest, followed by the concentration step change injection (CI) scheme, and that of the simultaneous injection (SI) scheme was the least. The visual flooding experimental results showed that the swept area of HPCF after waterflooding under the AI scheme was higher than that of the SI. Moreover, in view of qualitative analysis of remaining oil distribution, the EOR of AI of HPCF was higher than that of SI, which was consistent with the parallel sand-pack flooding results.

Selection of the particle size distribution of granular lost circulation materials for use in naturally fractured thief zones

Granular lost circulation materials (LCMs) are used to control fluid loss in naturally fractured thief zones. Rapid and automated identification of their optimal particle size distribution (PSD) is highly desirable. This paper proposes a PSD selection method for LCMs based on a PSD representation algorithm and the Monte Carlo method. The PSD representation algorithm describes the cumulative and compositional PSD curves of LCMs according to a piecewise cubic Hermite interpolation algorithm. Comparison with measured PSD data shows that the representation algorithm is more accurate than other commonly used PSD models in describing both homogeneous and mixed LCMs, and the relative errors are all within 10%. The Monte Carlo method is adopted to simulate LCMs with various compositions and select ones with PSDs that meet a proposed selection criterion. Experiments show that the PSD curves of the LCMs were all within the upper and lower bounds specified by the selection criterion. Plugging experiments show that LCMs with the recommended compositions can effectively enter fractures, and the fluid loss is less than 200 mL for half an hour under the pressure difference of 7 MPa. The proposed selection method can be used for the rapid and automatic design of granular LCMs used in fractured thief zones.

Dynamically Evaluating the Performance of Naturally Occurring Additives to Control Lost Circulation: On the Effect of Lost Circulation Material Type, Particle-Size Distribution, and Fracture Width

Summary Lost circulation is one of the most challenging problems during drilling of oil and gas wells. This issue leads to significant loss of drilling fluid, increase of nonproductive time as well as dictating additional costs to drilling companies. Lost circulation may also lead to other consequences, including stuck pipe, poor hole cleaning, and well control issues. How to efficiently control lost circulation have been traditionally depending on the type of the used lost circulation material (LCM). Injection of commonly available materials (without any further process on their chemical properties) into the thief zone is a common method of lost circulation control. These nonmodified materials are named as conventional LCMs against the unconventional LCMs which are designed/produced just for fluid lost control. The objective of this paper is to comparatively investigate the performance of cane, oak shell, wheat, and mica as LCM of water-based drilling fluid exposed to fractured formations. These materials were chosen because of their low cost, easy access, and compatibility with the environment. The sealing efficiency of these materials was assessed at different particle-size distributions (PSDs) for proper treatment of loss circulation. To do so, an experimental setup containing a cell with adjustable fracture size was designed. Among the LCM formulations made of each of the materials, oak shell formulations are better than the others, followed by mica and cane blends, respectively. The results reveal that combining the materials together is a better treatment than the separate use of them. As it will be seen in detail later, high diversity in particle size (broad PSD) causes more efficient control of fluid loss. Also, to reduce the dependency of sealing ability of LCM formulation on fracture size, mixing of the materials with different particle sizes and shapes is recommended.

MICROBIAL EOR STUDY TO IMPROVE SWEEP EFFICIENCY IN CALTEX FIELDS PHASE 1 – NUTRIENT SELECTION

The objective of this research is to find a cost effective nutrient that will stimulate the growth of in-situ microbes in Caltex Pacific Indonesia (CPI) oil fields to create sufficient biological products to plug high-permeability reservoir thief zones. If successful, injection fluids may be diverted into unswept regions of the reservoir, increasing the sweep efficiency und extending the production life of watered- out oil fields. This paper describes how a wide range of nutrients were researched and tested in the laboratory to achieve the objective. The primary findings of this study show that several primary nutrients were successful at creating bioproducts at high temperature, low salinity, reservoir conditions, and that molasses may be the most cost effective nutrient for all three CPI waterflooded reservoirs tested: Balam South, Bangko, and Minas fields. As a result of this work, laboratory studies were started to see to if the microbial growth created in laboratory test tubes can be recreated within reservoir core plugs to achieve sufficient permeability reduction to justify field trial (s). The core flood process and techniques will be presented in a separate paper.

Development Of Waterflood Profile Modification Using Brightwater Technology

Water flooding, in many mature fields is facing a common problem of low sweep efficiency in the late production period. The breakthrough of injection water is very early when high permeability streaks or thief zones exist in the formation, and resulted in excessive water production. Two kind of technologies commonly are used to modify the permeability streak i.e. MPM (Microbial Profile Modification), and polymer gel with cross linkers material. A new technology which is called Bright Water has been intensively studied. Bright Water is capable of in-depth placement into high permeability streaks in the reservoir. To improve the water flood sweep efficiency, studies of examination a fluid injection design have been evaluated. The objective of this study is to set-up core flooding tests and to determine the effective- ness of the Bright Water to reduce the permeability, and also include optimization of Bright Water formulation, resistance factor determination, and gelling time evaluation.

Tracer Tests For Heterogeneity Characterization And Saturation Determination On Core Flooding

Low sweep efficiency is the common problem in displacement process due to heterogeneity, high permeability streaks, fractures, and thief zones existing in the formation. Similarly, the success or failure of EOR implementations are always affected by those problems which causes displacing fluids fingering and early breakthrough. Factors of this type, unless properly identified and understood before the start of EOR process, will likely cause a project failure. Core flooding as the model of small scale of fluids movements in reservoir undergoes similar circumstances. Approximately one foot long of four 3.5 inches stacked native and synthetic cores are normally used in core flooding experiment. Tracer test was performed to characterize the core in addition of CT scan analysis. On this experiment, lithium solution was selected as tracer solution to be then injected into core at constant rate, 4 ft/day. Afterwards, the effluents were collected by Gilson sample collector in each tube for further determinination its concentration using Atomic Absorption Spectrometry (AAS). Response curves of lithium tracer were able to determine core heterogeneities and this should be done to avoid misleading interpretation of core flooding results. Besides, lithium concentration reported in some extent and subsequently analyzed by employing method of temporal moments. This method provides numerical calculation to estimate effective core pore volume (PV) and fluid saturation. Weighing method was also used to compare the PV with aforementioned method and the results were comparable.

Effects of thief zones on displacement efficiency: Microfluidic pore-scale and conformance control analysis

Thief zones are regions with high permeability, usually formed in mature or depleted reservoirs after water flooding, which can further reduce the overall recovery during subsequent EOR stages. While existing conformance control solutions alleviate the impact of thief zones, selecting an optimal solution based on current laboratory methods is challenging, since current methods fail to isolate the impact of thief zones, lack tight control of initial conditions, and use long evaluation times as well as large sample volumes. In this study, a microfluidic analogue is designed and fabricated with multiple permeability regions, representing thief zones in a reservoir, to evaluate the performance of polymer flood in a porous medium. The analogue contains four connected and distinct heterogenous zones of different permeability, in which the fluid dynamics and relevant conformance control are resolved during secondary and tertiary polymer floodings, once thief zones are apparent. The secondary water flooding did not exhibit an efficient displacement, with fingering and by-passing effects predominantly observed in higher permeability zones and minor oil displacement in the lowest permeability zone. Despite the fingers created by water flooding, polymer flooding showed much higher displacement efficiency throughout all four zones, resulting in significant incremental oil recovery. The secondary polymer flooding also produced more oil than the secondary water flood due to improvement in the mobility control. Lastly, pore-scale mechanisms such as viscous cross-flow between zones was observed during the polymer flood.

Influence of top water on SAGD steam chamber growth in heavy oil reservoirs: An experimental study

Steam-assisted gravity drainage (SAGD) is one efficient and mature technology for recovering heavy oil and bitumen resources. The key underlying mechanism is the growth of the steam chamber after injecting steam. However, due to the complex geological environment, the thief zones exist and have a prejudicial effect on the development of the steam chamber, thus impacting the ultimate heavy-oil recovery. In this work, our objective is to investigate the effect of a top-water thief zone (i.e., water zone overlies the oil sand) on SAGD performance and further to understand the crucial mechanisms that control the heat loss during steam injection. A large-scale three-dimensional experimental apparatus is used to carry out the SAGD process with a top aquifer. Based on the similarity criterion, the field-scale model is transformed into a laboratory elemental model. To evaluate the SAGD performance quantitatively, the dynamic growth of the steam chamber is measured using the thermal detectors and the production data is recorded. The results show that the steam chamber exhibits three distinguished stages, that is, upward spread, lateral extension, and downward development in the presence of top-water zone. The bottom-water zone has less impact on the steam-chamber growth. The existence of a confined top-water zone, however, significantly affects SAGD performance, especially the lateral expansion of the steam chamber. The lateral propagation of the steam front is hindered by the top thief zone due to the heat exchange with the top water. Once the steam chamber reaches the boundary, the accumulation of energy in the water thief zone, in turn, can reduce the remaining oil saturation along the topwater–oil interface. This study provides us some key insights into the development of heavy oil resources with top thief zones when implementing SAGD technology.