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Summary Rock typing into flow units (FUs) plays a pivotal role in constructing static and dynamic models of petroleum reservoirs. Decisions made by asset teams mostly depend on predictions of how fluids will percolate through the subsurface during the reservoir life cycle. In carbonate settings, dealing with rock typing is complex and can generate a large quantity of units because of diagenetic processes such as dissolution, cementation, and silicification. Seismic data can be used to detect large-scale FUs and assist the interpolation of small-scale FUs in 3D reservoir volume, thus producing more-realistic static and dynamic models. We propose a modification of the classical rock-typing methods that use permeability (k) vs. porosity (/) plots and electrical properties, with a data set from the Mero Field, part of the giant Libra Field of presalt carbonate reservoirs in offshore Brazil. From the permeability cumulative S-curve analysis, we established major large-scale FUs that maintain part of the carbonate flow heterogeneity and correlate them with the elastic attributes: P-impedance (PI) and S-impedance (SI). In addition, we established a priori PI and SI correlations with the formation-factor (FF) (F) parameter to categorize large-scale FUs using the F vs. k methodology. With the large-scale FUs mapped in seismic data sets, core-plug-scale FUs can be populated into the 3D static and dynamic models using geostatistics tools, thus creating more-realistic reservoir models and assisting asset teams in the decision-making process. Introduction Hydrocarbon reservoirs are heterogeneous and not uniform, and can be divided into multiple homogeneous groups called FUs. Each unit presents similarities in terms of grain size, texture, cementation, pore distribution, porosity, and other physical characteristics controlled by the sediment depositional environment and diagenesis (Altunbay et al. 1994). FUs for reservoir characterization are an effective way to simulate fluid movement and oil-production behavior. According to Ebanks (1987), "a flow unit (FU) is a representative elementary volume of the total reservoir rock, within which geological and petrophysical properties that affect fluid flow rate are internally consistent and predictably different from properties of other rock volumes." Rock typing for FUs in reservoirs has been a source of debate for geoscientists.
Ba Geri, Mohammed (Missouri University of Science and Technology) | Ellafi, Abdulaziz (University of North Dakota) | Flori, Ralph (Missouri University of Science and Technology) | Belhaij, Azmi (Saudi Metal Coating Company) | Alkamil, Ethar H. K. (University of Basrah)
Nowadays, as the worldwide consumption of hydrocarbon increases, while the conventional resources beings depleted, turning point toward unconventional reservoirs is crucial to producing more additional oil and gas from their massive reserves of hydrocarbon. As a result, exploration and operation companies gain attention recently for the investment in unconventional plays, such as shale and tight formations. A recent study by the U.S. Energy Information Administration (EIA) reported that the Middle East (ME) and North Africa (NF) region holds an enormous volume of recoverable oil and gas from unconventional resources. However, the evaluation process is at the early stage, and detailed information is still confidential with a limitation of the publication in terms of unconventional reservoirs potential. The objective of this research is to provide more information and build a comprehensive review of unconventional resources to bring the shale revolution to the ME and NF region. In addition, new opportunities, challenges, and risks will be introduced based on transferring acquiring experiences and technologies that have been applied in North American shale plays to similar formations in the ME and NF region. The workflow begins with reviewing and summarizing more than 100 conference papers, journal papers, and technical reports to gather detailed data on the geological description, reservoir characterization, geomechanical property, and operation history. Furthermore, simulation works, experimental studies, and pilot tests in the United States shale plays are used to build a database using the statistic approach to summarize and identify the range of parameters. The results are compared to similar unconventional plays in the region to establish guidelines for the exploration, development, and operation processes. This paper highlights the potential opportunities to access the unlocked formations in the region that holds substantial hydrocarbon resources.
Ba Geri, Mohammed (Missouri University of Science and Technology) | Flori, Ralph (Missouri University of Science and Technology) | Ellafi, Abdulaziz (University of North Dakota) | Noles, Jerry (Coil Chem LLC) | Essman, Jacob (Coil Chem LLC) | Kim, Sangjoon (Coil Chem LLC) | Alkamil, Ethar H. K. (University of Basrah)
Hydraulic fracturing operation requires securing sufficient water resources to access unlocked formations. Successful treatment depends on the fracture fluids that mainly consists of water-based fluid with a low percentage of chemical additives around 1%. Therefore, the oil and gas industry are considered as the largest freshwater consumers by 3 to 6 million gallons of water per well based on a number of fracturing stages. As a result, the traditional water resources from subsurface and surface are getting depleted, and availability of freshwater is becoming more difficult with high cost due to continued demand. For example, operator companies in West Texas face many challenges, including a recent increase from USD 3 to 10 per m3 of freshwater. In addition, transporting process of the raw water to the fracture sites, such as Bakken has an environmental impact, and expensive costs up to USD 5/bbl, while costs of water disposal in range of USD 9/bbl.
This paper aims to study the produced water as alternative water-based fluid with high viscosity friction reducers (HVFR) to reduce environmental footprints and economic costs. To address utilizing produced water as an alternative capable water resource that may use during fracturing treatment, this research presents an experimental investigation associated with using the Permian high-TDS brine water with HVFRs. This work includes experimental research, case studies, and guidelines work on recent improvements on using HVFR to carry proppant and capture the optimum design in fracturing operations. Moreover, the research conducted scaled lab friction measurements that can in turn to be used to improve forecasting of frictions in the field, and therefore of expected surface treating pressures during fracture treatments. Evaluating pipe friction as a function of time to compare HVFRs efficacy in lab and field conditions as well as to predict maximum injection rate during a frac job is investigated.
The outcomes show that high-TDS Permian water with highest dosage of HVFRs had instantaneous pressure reduction effect in 10 seconds while low dosage of HVFRs had lost the effect slowly after 4 min. 30 sec. Also, the results of this study show that the variation of viscosity and pressure reduction at higher shear rate is small. The warm temperature helped rapid polymer dispersion and provided better environment to polymer hydration leads to rapid pressure reduction. Finally, successful implementation in Wlofcamp formation shows that the operation treating pressure reduced from 11,000 to 8,000 psi. The general guidelines obtained can promote the sustainability of using hydraulic fracturing treatment to produce more oil and gas from unconventional resources without considering environmental issues.
Rock classification plays significant role in determining the fluid flow movement inside the reservoir. With recent developments in computer vision of porous medium and artificial intelligence techniques, it is now possible to visualize unprecedented detail at the scale of individual grains, understand the patterns of contact angles and its direct connection to multiphase fluid movements within the porous media. The outcome of this work is a probabilistic rock classification model that provides a reliable and realistic description of the reservoir.
As part of this work, 400 fully brine saturated 3D micro-CT images of Bentheimer and Clashach micro core plugs are utilized. Various three-dimension image analysis techniques are applied to quantify the rock properties (e.g. porosity, absolute permeability) and to extract pore structure information, such as pore throat distribution, pore connectivity and pore roughness from these images. The rock surface roughness is quantified as the local deviation from the plane (AlRatrout et al. 2018). The whole image dataset is divided into two separate subsets, 80% for training purpose and 20% for testing purpose. Both subsets are fed to an artificial intelligence-based model to verify and validate the results. To improve the accuracy of the model, k-fold validation technique is implemented.
The accuracy of the developed model is validated using Root-Mean-Square Error (RMSE), coefficient of determination (R2) and relative error (RE). Blind test of comparing predicted results with second subset of experimental data have shown that the developed model is capable to predict rock type with a maximum error of 3.5%. The results of this study indicate that for the given dataset, pore surface roughness has dominant effect on rock classification.
The accuracy of the developed model can be improved by incorporating additional information, for example rock mineralogy. However, the developed model is limited only aforementioned rock types, can be easily extended to other rock types provided enough micro CT images are available.
Al-Maqtari, Ameen N. (SAFER E&D Operations Company) | Saleh, Ahmed A. (SAFER E&D Operations Company) | Al-Haygana, Adel (SAFER E&D Operations Company) | Al-Adashi, Jaber (SAFER E&D Operations Company) | Alogily, Abdulkhalek (SAFER E&D Operations Company) | Warren, Cassandra (Schlumberger) | Mavridou, Evangelia (Schlumberger) | Schoellkopf, Noelle (Schlumberger) | Sheyh Husein, Sami (Schlumberger) | Ahmad, Ammar (Schlumberger) | Baig, Zeeshan (Schlumberger) | Teumahji, Nimuno Achu (Schlumberger) | Thiakalingam, Surenthar (Schlumberger) | Khan, Waqar (Schlumberger) | Masurek, Nicole (Schlumberger) | Andres Sanchez Torres, Carlos (Schlumberger)
A 3D petroleum systems model (PSM) of Block 18 in the Sab'atayn basin, onshore western Yemen, was constructed to evaluate the untapped oil and gas potential of the Upper Jurassic Madbi formation. 3D PSM techniques were used to analyze petroleum generation for conventional reservoirs and the petroleum saturations retained in the source rock for the unconventional system. Block 18 has several proven petroleum systems and producing oil and gas fields. The principal source rocks are within the Madbi Formation, which comprises two units, the Lam and the Meem members. Both contain transgressive organically rich "hot" shales with total organic carbon (TOC) of 8 to 10%; these are located stratigraphically at the base of each member. Additional organic-rich intervals within the Lam and Meem are less-effective source rocks, with lower TOC values.
The PSM consisted of 17 depositional events and 2 hiatuses. To accurately replicate geochemical and stratigraphic variations, the Lam and Meem members were further divided into sublayers. The model was calibrated to present-day porosity, permeability, and pressure data, and it incorporated vertical and lateral lithofacies and organic facies variations. Further calibrations used observed maturities (vitrinite reflectance and pyrolysis Tmax) and present-day temperatures and considered laterally variable heat flow from the Early Jurassic to the Late Miocene. Finally, petrophysical analyses from wells provided calculated hydrocarbon saturations, which were used to calibrate the saturation output from the model. The model satisfactorily reproduces the distribution of the main gas and oil fields and discoveries in the study area and is aligned with well test data.
Maturity results indicate that the upper Lam intervals currently sit within the main to early oil window but are immature at the edges of Block 18 (based on the Sweeney and Burnham Easy R0% kinetics). The lowest Lam unit enters the wet gas window in the center of the block. The underlying Meem member ranges from wet gas to early oil window maturity. Like the Lam, the Meem remains immature along the edges of Block 18. However, in the south of the block, the richest source rocks within the Meem are mainly in the oil window. The degree of transformation of the Meem and Lam varies throughout the members. The model predicts that, at present, the lowest part of the Meem, containing the greatest TOC, has 90% of its kerogen transformed into hydrocarbons.
The model confirms that the Madbi formation is a promising unconventional shale reservoir with a high quantity of hydrocarbons retained within it. Despite the higher quantity of hydrocarbons retained in the upper Meem, in terms of liquid and vapor hydrocarbons predicted in this model, the lower Lam is the most-prospective conventional tight sand reservoir, and the Meem has very small potential as tight sand reservoirs. This study provided a novel application of 3D PSM technology to assess new unconventional as well as conventional plays in this frontier area.
Ba Geri, Mohammed (Missouri University of Science and Technology) | Ellafi, Abdulaziz (University of North Dakota) | Flori, Ralph (Missouri University of Science and Technology) | Noles, Jerry (Coil Chem LLC) | Kim, Sangjoon (Coil Chem LLC)
Viscoelastic property of high-viscosity friction reducers (HVFRs) was developed as an alternative fracturing fluid system because of advantages such as the ability to transport particles, higher fracture conductivity, and potential lower cost due to fewer chemicals and equipment on location. However, concerns remain about using HVFRs to transport proppant in DI water and harsh brine solution (e.g. 2wt% KCl and 10 lbs. brine). The primary objective of this study is to investigate the viscoelastic property that can help to understand the true proppant transporting capacity of fracturing fluids in high-TDS environment.
To address the evaluation performance of HVFRs, a comprehensive review of numerous papers associated to viscoelastic property of hydraulic fracturing fluids were investigated and summarized. This paper also provides a full comparison study of viscosity and elastic modulus between HVFRs and among fracturing fluids such as xanthan, polyacrylamide-based emulsion polymer, and guar. Moreover, viscosity profiles and elastic modulus were conducted at different temperatures. Better proppant transportation effect though higher viscosity through Stoke's law and the effect on proppant transportation from elastic modulus comparison were also investigated. Finally, HVFR Conductivity test and successful field test result were explained.
The results of the experimental work show that viscoelastic property HVFRs provides good behavior to transport proppant. Viscosity profile decreased slightly as the temperature increased from 75 to 150 when the DI water was used. While using 10 lbs. Brine the viscosity was reduced by 33%. The longer polymer chains of HVFR indicated better elastic modulus in DI water. The elastic modulus also indicated that the highest values at frequency 4.5 Hz from each amplitude, and lower values as amplitude was increased. Although high molecular weight HVFRs were utilized on the conductivity test, the results observed that the regained permeability was up to 110%. Finally, the promising results from the case study showed that using HVFRs could be performed economically and efficiently for the purpose of proppant transportation and pressure reduction in high TDS fluids.
This challenging reservoir characterization case study is defined by the interaction between two reservoirs with different production mechanisms: a fractured basement reservoir and an overlying sandstone reservoir. The existing static geologic concept has been significantly enhanced by integrating pressure data from a unique three-year shut-in period to aid modeling of fractured reservoir connectivity. Previously, the seismic dataset was predominantly used to model the fault and fracture network and guide well planning. In the current approach, the full field data set, including all drilling parameters and new reservoir surveillance data were integrated to address uncertainty in the connected hydrocarbon volume and the relative importance of each production mechanism. The result is a reservoir management tool with which to test re-development concepts and effectively manage pressure decline and increasing gas/oil ratio (GOR) and water production.
To achieve a fully integrated history matched model, the first step was to make a thorough review of the existing detailed seismic interpretation, vintage production logging tool runs (PLT's), wireline logs (including borehole image logs (BHI)) and drilling data to find a causal link between hydraulically conductive fractures and well production behavior. In parallel, a material balance exercise was run to incorporate the new pressure data acquired during the field's shut-in period. The results of the material balance analysis were combined with seismic and well data to define the distribution of connected fractures across the field. Additionally, the material balance analysis was used to determine the connected hydrocarbon volume, the distribution of initial oil in-place and the relative hydrocarbon contribution from each production mechanism.
The field is covered by multi-azimuth 3D seismic and 43 vertical to highly deviated development wells, providing significant static and dynamic data for characterizing the distribution of connected fractures. Despite this high quality, diverse and field-wide dataset, prior modeling iterations struggled to sufficiently describe the production behavior seen at the well level. This has resulted in a major challenge to predicting the production behavior of new development wells and planning for reservoir management challenges. Capturing the complex interaction between production variables (including lithology, matrix versus fracture network, geomechanical stresses, reservoir damage and pressure depletion) at a field level instead of at an individual well level resulted in a unified static and dynamic model that reconciles all scales of observation.
This oilfield represents a unique reservoir characterization opportunity. The result is a key example of how iterative, integrated geological and engineering driven reservoir modeling can be used to inform the development in a complex, mature field. This case study provides an excellent analogue for the reservoir characterization of other fractured Basement fields and/or Basement-cover reservoir couplet fields in the early to late phases of their development.
High-viscosity friction reducers (HVFRs) have been gaining popularity and increase in use as hydraulic fracturing fluids because HVFRs exhibit numerous advantages such as their ability to carry particles, their promotion of higher fracture conductivity, and their potentially lower cost due to fewer chemicals and equipment on location. However, concerns remain about using HVFRs with produced water containing a high level of TDS (Total Dissolved Solids). This study investigates the influence of the use of produced water on the rheological behavior of HVFRs compared to a traditional linear guar gel. This work also aims to correlate proppant settling velocity behavior with rheological properties of HVFRs vs. linear gel on hydraulic fracturing operations. Comprehensive rheological tests of different HVFRs compared with linear gel were performed including, shear-viscosity and dynamic oscillatory-shear measurements using an advanced rheometer.
The results of these rheological measurements reveal that these polyacrylamide-based HVFR systems achieve a high viscosity profile in fresh water with associated high proppant-carrying capacity. On the other hand, increasing water salinity lowers HVFR’s viscosity, increases proppant settling velocity, and lessens the fluid’s proppant-carrying efficiency. Although in fresh water linear gel showed similar viscosity measurements with HVFR-A, the HVFR-A recorded a lower proppant settling rate because HVFR-A has a higher relaxation time (15.3 s) than the relaxation time of linear gel (1.73 s).
As expected, in high-TDS produced water the relaxation time and elastic behavior decreased for all the fracturing fluids tested. HVFR-B recorded the smallest reduction in relaxation time (about 14%) when tested in produced water vs. fresh water, and the resulting settling velocity increased by 29% from 3.4 cm/s to 4.85 cm/s. For linear gel, its reduction in relaxation time exceeded of 70% when changing water salinity from fresh water to high-TDS brine water. This high reduction of relaxation time leads to over 40% increase in proppant settling velocity from 5.3 cm/s to 8.7 cm/s in fresh water and produced water, respectively. This study confirms that HVFR’s elasticity (vs. it viscosity) properties enable successful proppant transport for a wide range of shear rates while viscosity (vs. elasticity) properties controls proppant settling velocity in linear guar-based fluids. This paper will provide greater understanding of the importance of complete viscoelastic characterization of the HVFRs. The findings provide an in-depth understanding of the behavior of HVFRs under high-TDS brine, which could be used as guidance for developing fracturing fluids and for fracture engineers to design and select better friction reducers.
ABSTRACT: Basement fractured reservoirs have proven to yield significant contributions of hydrocarbon production in several countries. Sab'atayn Basin in Yemen, has potential production and is classified as an unconventional reservoir. Due to poor reservoir quality from low porosity and ultra-low permeability, understanding the petrophysical properties and geomechanical characterization can lead to optimize the design for hydraulic fracturing treatments. The workflow in this research started by evaluating the formation of interest, then building the geomechanical model to assess the three different fracturing fluid scenarios for hydraulic fracture modeling. The results showed hydrocarbon potential in the fractured oil-bearing zone with a dominated fracture porosity of almost 2.2% and a high amount of shale content. As a new study area, the geomechanical property results are compatible within the typical range of several basement fractured reservoirs worldwide. As a fracturing fluid, produced water is the appropriate fluid treatment in terms of creating a high fracture half-length with low damage and environmental footprint. However, the high viscosity friction reducer fluid has more potential to transport the proppant deeper into the fracture. The research findings provide a deep understanding of geomechanical models, which could be used as guidance for fracture engineers to design and optimize fracturing treatments.
Natural Fractured Reservoirs (NFRs) are defined as formation rocks that are characterized by a series of discontinuous fractures/faults/fissures/or bedding planes, and their lithology can be carbonates, sandstones or crystalline rocks. Several researchers reported that these reservoirs have been successfully proven to be a significant contribution to hydrocarbon production, since a large proportion of produced hydrocarbon worldwide are NFRs (Nelson, 1985; Aguilera, 1996; Badakhshan et al., 1998; Nelson, 2001; Rodriguez et al., 2004; Nicolas et al., 2011). However, the detection and evaluation processes for these sweet spots are challenging for geologists, geophysicists, and petroleum engineers due to heterogeneity in the pore structure of the rock (El Sharawy, 2015). In the last decade, discovery and development in these reservoirs has increased rapidly due to advances in horizontal drilling and hydraulic fracturing technologies. Landes et al., 1960 and Aguilera 1996 defined fractured basement formations as metamorphic or igneous rocks (regardless of age) which are unconformably overlain by a sedimentary sequence. As a result, faults were led to create fracture networks and pore space through diagenetic processes. Furthermore, hydrocarbon was formed and stored in the natural fractures due to tectonic activity. In addition to sandstone or carbonate formations, the basement reservoirs are usually found in the lower zone of the oil-bearing formations with a significant amount of hydrocarbons that accumulated in the natural fracture between the rock matrix. These resources are considered to have a sedimentary origin, which are fractured quartzite or granites (North, 1990; Koning 2013). In 1979, Nelson characterized the NFRs into four different categories; most of the basement formation fall into the category that the reservoirs have relatively low permeability and no or low matrix porosity. Therefore, these reservoirs are classified as unconventional resources that are the primary target to produce additional oil and gas (Nicolas et al., 2011; Pascal and Priscilla, 2017). There has been little exploration conducted on this complex reservoir type, fractured basement granite rocks are attractive to the oil and gas industry because of their popularity in more than 30 different countries, such as Algeria, China, Vietnam, Canada, India, Yemen, UK, Libya, and Egypt (Sircar 2004; Gutmanis, 2009). Since 1975, Vietnam has large discoveries, where the Bach Ho oil field has estimated two billion barrels of oil reserves in the Cuu Long Basin's offshore field (Keggin and Alaaraji, 2017). In contrast, Yemen had a large onshore exploration in last 20 years, where ten blocks of basement fractured granite reservoirs were detected in the Masilah and the Sab'atayn Basins. Because of the poor reservoir quality (low porosity and ultra-low permeability), only five of the ten blocks are producing, and East Shabwa has the highest daily oil production around 11,765 m3/day (74 M Rbbl/day) (Nicolas et al., 2011; Bawazer et al., 2018). This case study emphases understanding the petrophysical properties and geomechanical characterization of fractured granite basement reservoirs as an unconventional resource in the Middle East region to gain insight into the optimum design for hydraulic fracturing treatments.
Witte, Jan (Falcon Geoconsulting) | Trümpy, Daniel (DT EP Consulting) | Meßner, Jürgen (Federal Institute for Geosciences and Natural Resources ) | Babies, Hans Georg (Retired from Federal Institute for Geosciences and Natural Resources )
Several wells have encountered good oil shows in the rift basins of northern Somalia, however, without finding commercial hydrocarbons to date. It is widely accepted that these basins have a similar tectonic evolution and a comparable sedimentary fill as the highly productive rift basins in Yemen from which they have been separated by the opening of the Gulf of Aden (fully established in Mid Oligocene). We present new regional tectonic maps, new basement outcrop maps, a new structural transect and new play maps, specifically for the Odewayne, Nogal, Daroor and Socotra Basins.
Digital terrain data, satellite images, surface geology maps (varying scales), oil seep/slick maps, potential data (gravity), well data from ~50 wells and data from scientific publications were compiled into a regional GIS-database, so that different data categories could be spatially analyzed.
To set the tectonic framework, the outlines of the basins under investigation were re-mapped, paying particular attention to crystalline basement outcrops. A set of play maps was established. We recognize at least three source rocks, five reservoirs and at least three regional seals to be present in the area (not all continuously present). Numerous oil seeps are documented, particularly in the Nogal and Odewayne Basins, indicative of ongoing migration or re-migration. Data from exploration wells seem to further support the presence of active petroleum systems, especially in the central Nogal, western Nogal and central Daroor Basins.
Our GIS-based data integration confirms that significant hydrocarbon potential remains in the established rift basins, such as the Nogal and Daroor Basins. Additionally, there are a number of less known satellite basins (on and offshore) which can be mapped out and that remain completely undrilled. All of these basins have to be considered frontier basins, due to their poorly understood geology, remoteness, marketing issues and missing oil infrastructure, making the economic risks significant. However, we believe that through acquisition of new seismic data, geochemical analysis, basin modelling and, ultimately, exploration drilling these risks can be mitigated to a point where the economic risks become acceptable.
We encourage explorers to conduct regional basin analysis, data integration, a GIS-based approach and modern structural geology concepts to tackle key issues, such as trap architecture, structural timing, migration pathways and breaching risks.