In the Midland Basin of west Texas, produced water volumes have historically been disposed into shallow intervals (i.e., Grayburg-San Andres). Over the last decade, the rapid growth in unconventional resource development has resulted in a significant increase in the volume of produced water leading to pressure gradient differences between shallow disposal zones and deeper intervals. These conditions have created drilling challenges and have prompted operators to test additional zones suitable for produced water disposal. In recent years, the Early Ordovician Ellenburger (ELBG) reservoir has become an alternative disposal interval to shallower reservoirs.
The Ellenburger Group of west Texas, a prolific producing reservoir, is part of an extensive carbonate system best known for karst development associated with prolonged subaerial exposure and intervals of high secondary porosity in fracture breccias generated by subsequent cave collapse. Many authors have described fracture occurrence and karst-related breccias of the ELBG, both of which impact productivity at the reservoir scale within the fields and make regional correlations particularly challenging. Ellenburger depositional facies have been described by previous workers in equivalent units across west and central Texas, and textural analysis of high-resolution electrical borehole images from recently drilled disposal wells, combined with core observations, shows corresponding porous intervals to be present in the Midland Basin.
This paper describes the generation of a regional model of porosity distribution within the Ellenburger and assesses the important differences in depositional environment and diagenetic history that exist among the internal units of the ELBG that may impact salt water disposal (SWD) well performance. For example, the Upper ELBG is dominated by fracture porosity in breccia fabrics associated with collapsed cave systems, while the Lower ELBG exhibits preserved porosity associated with original depositional textures. The regional model was tested using multiple datasets: image logs, core descriptions, electric logs from more than 400 well penetrations, and injection data from recent well tests. The integration of these datasets has resulted in a suite of maps of the key stratigraphic intervals within the ELBG that offer the greatest potential for disposal. Additionally, the integration of well performance with observed regional geologic trends was used to identify and tier key performance drivers for deep SWD injection performance, resulting in refined performance maps that can be used for strategic placement of deep SWD wells.
Eastern dipping carbonate and evaporite outcrops of Upper Permian formations in the western Delaware Basin facilitate the meteoric recharge of a minor saline aquifer. Through time this recharge has created karst features, which are a source of shallow drilling hazards within the first thousand feet below ground level. When encountered during drilling, if these features are big enough, they can cause loss of drilling fluid, wellbore instability, bit drops, and are a potential risk to surface equipment. Detecting and avoiding these shallow drilling hazards is imperative to continued safe and cost-effective operation. Here we present an integrated approach to detect and avoid shallow drilling hazards through the utilization of various technologies, such as remote sensing, seismic, airborne gravity and resistivity surveys.
During the pre-staking process we examine surface and subsurface conditions to select a potential drilling pad, and surface hole locations using high-resolution optical imagery, LiDAR, satellite imagery, airborne gravity, and seismic data. Detailed subsurface karst detection is then performed in the field around proposed surface hole locations, utilizing high-resolution ground resistivity surveys. Each high-resolution ground resistivity survey consists of several resistivity lines spaced at a distance necessary to resolve potential features between surface hole locations. The orientation of resistivity grids is defined based on airborne gravity anomaly trends and obstacles detected on the ground surface. Anomalies mapped through measured resistivity profiles are used to predict cavities and infilled materials. If significant air-filled karsts are detected, surface hole locations are adjusted, and the high-resolution ground resistivity process continues, until detectable drilling-hazards are avoided.
We have tested and validated our integrated workflow at existing drilling pads where karst features were confirmed through drilling. This workflow is being applied across our unconventional developments with similar near-surface lithology, in Culberson, Reeves, and Loving counties, Texas.
Work demonstrates a collaborative effort between Geologists, Geophysicists, Remoting Sensing Specialists, Well Planners, and Drilling Engineers to enhance operational efficiencies across our Delaware Basin assets.
The significant oil reserves related to karst reservoirs in Brazilian pre-salt field adds new frontiers to the development of upscaling procedures to reduce time on numerical simulations. This work aims to represent karst reservoirs in reservoir simulators based on special connections between matrix and karst mediums, both modeled in different grid domains of a single porosity flow model. This representation intends to provide a good relationship between accuracy and simulation time.
The concept follows the Embedded Discrete Fracture Model (EDFM) developed by Moinfar, 2013; however, this work extends the approach for karst reservoirs (Embedded Discrete Karst Model - EDKM) by adding a representative volume through grid blocks to represent karst geometries and porosity. For the extension of EDFM approach in a karst reservoir, we adapt the methodology to four stages: (a) construction of a single porosity model with two grid domains, (b) geomodeling of karst and matrix properties for the corresponding grid domain, (c) application of special connections through the conventional reservoir simulator to represent the transmissibility between matrix and karst medium, (d) calculation of transmissibility between karst and matrix medium.
For a proper validation, we applied the EDKM methodology in a carbonate reservoir with mega-karst structures, which consists of non-well-connected enlarged conduits and above 300 mm of aperture. The reference model was a refined grid with karst features explicitly combined with matrix facies, including coquinas interbedded with mudstones and shales. The grid block of the reference model measures approximately 10 × 10 × 1 meters. For the simulation model, the matrix grid domain has a grid block size of approximately 100 × 100 × 5 meters. The karst grid domain had the same block size as the refined grid. Flow in the individual karst grid domain or matrix grid domain is governed by Darcy's equation, implicitly solved by simulator. However, the transmissibility for the special connections between karst and matrix blocks is calculated as a function of open area to flow, matrix permeability and block center distance. The matrix properties were upscaled through conventional analytical methods. The results show that EDKM had a considerable performance regarding a dynamic matching response with reference model, within a reduced simulation time while maintaining a higher dynamic resolution in the karst grid domain without using an unconstructed grid.
This work aims to contribute to the extension of EDFM approach for karst reservoirs, which can be applied to commercial finite-difference reservoir simulators and it presents itself as a solution to reduce simulation time without disregarding the explicit representation of karst features in structured grids.
Wang, Hongqiu (PetroChina Research Institute of Petroleum Exploration & Development-Northwest, Lanzhou, Gansu, China) | Liu, Weifang (PetroChina Research Institute of Petroleum Exploration & Development-Northwest, Lanzhou, Gansu, China) | Gao, Jianhu (PetroChina Research Institute of Petroleum Exploration & Development-Northwest, Lanzhou, Gansu, China) | Li, Shengjun (PetroChina Research Institute of Petroleum Exploration & Development-Northwest, Lanzhou, Gansu, China) | Gui, Jinyong (PetroChina Research Institute of Petroleum Exploration & Development-Northwest, Lanzhou, Gansu, China) | Guo, Xin (PetroChina Research Institute of Petroleum Exploration & Development-Northwest, Lanzhou, Gansu, China)
Two sorts of ancient water patterns that surface water pattern and underground water pattern exist and develop near the Ordovician carbonate buried hill in Tarim Basin. Frequency mixing technique can effectively improve the resolution effect of the frequency division result attribute of single frequency, which is more effective in detailed identifying the surface and underground river. Both surface river pattern and underground river pattern can form effective karst space, which provides important proof for revealing the development law of the Ordovician ancient karst in Tarim Basin. Underground river pattern controls the distribution of large karst cavity, and poses significant impact on the formation of top large collapse fractures due to cavity collapse in later stage; surface river pattern has mainly invasion effect at the buried hill high parts and lateral and vertical corrosion at low parts, however, deep corrosion can be formed on both sides and riverbed. During the process of diagenesis, fractures are formed due to later filling and overburden differential compaction. The study on ancient river pattern poses significance for revealing karst distribution law, deepening non-tectonic fracture study and expanding exploration of fractured reservoirs.
Presentation Date: Monday, October 15, 2018
Start Time: 1:50:00 PM
Location: Poster Station 5
Presentation Type: Poster
Shallow hazards associated with dissolution karsting within the Castile formation of the Delaware basin of west Texas represent a significant drilling hazard. One operator encountered four karst related cavities within the first four hundred feet of section from two separate pad sites. The intersection of these dissolution cavities resulted in the loss of significant amounts of drilling fluids and concrete, resulting in the abandonment of the boreholes. Approximately 1100 line-km of Airborne Full Tensor Gradiometry (FTG) data are acquired at a nominal line spacing of 100 m and drape elevation at 80 m above the ground surface in order to rapidly identify and assess further risks to drilling in the adjacent lease acreage. 2.75D and 3D models are constructed to determine detectability and identify relevant wavelength and amplitudes based upon information collected from the karst cavities. Focused enhancements of the acquired data reveal correlation to the drilled cavities, but the wavelengths are too broad to assess drilling risks. Curvature analysis of the FTG data reveal a striking correlation to cavern analysis performed in outcropping areas of the Castile formation. Riskiest and safest drilling sites are identified by means of analyzing the curvature information.
Presentation Date: Tuesday, October 16, 2018
Start Time: 8:30:00 AM
Location: 207A (Anaheim Convention Center)
Presentation Type: Oral
Lambert, Mark (Tatweer Petroleum Bahrain LLC) | AlMuftah, Ali (Tatweer Petroleum Bahrain LLC) | Yousif, Maysa (Tatweer Petroleum Bahrain LLC) | Barni, Sajeda (Tatweer Petroleum Bahrain LLC) | Ai, Guang (Tatweer Petroleum Bahrain LLC) | Zhang, Eamon (Occidental Petroleum)
The Late Cretaceous Mishrif limestone is known locally in Bahrain as the "Rubble" due to extensive tectonic fracturing and karst brecciation which may have tens to hundreds of times the permeability of the tight matrix where most oil resides. If these secondary features are not properly managed, steam utilization during thermal heavy oil recovery is compromised, so heat transfer to viscous crude in the matrix is less effective. To address this challenge, field pilots are conducted across Awali field to apply customized thermal processes to varying geology.
Designing the thermal pilots uses 3D static and dynamic models representing the heterogenous reservoir to test various thermal processes (cyclic steam stimulation, forced imbibition, and steamflood). Initial geocellular models are multi-well secular models utilizing simplified erosional surface truncations and fault-fracture networks to optimize run times for multi-component thermal simulation. Reservoir properties are guided by well log data and geostatistics, whereas an effective permeability hierarchy accounts for historic well performance and observations. Additional inputs for simulation include thermal properties for rock and fluids, temperature-dependent relative permeability curves, temperature-viscosity relationships and vapor-liquid equilibrium ratios for each crude component, and injected steam quality.
Piloting in a sparsely fractured portion of Awali field gave unexpected results that required revisiting conceptual geologic models and testing new ideas with dynamic simulation. This effort was accelerated by multiple scenario playing with "pseudo-fractures" strategically relocated to honor dynamic data while remaining consistent with the known fault, fracture and karst trends.
This paper describes the results of this particular pilot and the versatile modeling approach taken to understand the outcomes. It recommends early quick multiple scenario playing to help guide subsequent time-intensive detailed fault-fracture-karst modeling.
The ALBION project applies a new and disruptive methodology of reservoir characterisation to the carbonate Urgonian Formation (South-East France) considered as the very best analogue of Mid Cretaceous reservoirs from Middle East. Thanks to numerous field sections and outcrops descriptions, to tens of wells drilled in the reservoir, to kilometres of cores, to monitoring of groundwater dynamics such as decades of hydraulic observations at pretty much the only natural outlet of a major groundwater reservoir (Fontaine-de-Vaucluse spring) and to a unique underground laboratory (LSBB, about four kilometers in the heart of the reservoir), a multi-scale model is being built for reservoir purpose. Different observation sites with wells whose spacing ranges from 2 to 20 meters contribute to the assessment of together the matrix, the fractures and the karst flow behaviours. Through the building of an observatory in the heart of a reservoir, the ALBION project is delivering advanced concepts and methodologies to apply to industrial projects in Middle East carbonate fields.
X Field is a carbonate platform setting in Central Luconia, Malaysia. The field faced declining trend of gas production due to early water rise. Seismic attribute shows that the rising water was assumed to be related to the water being drained through crossflow into the possible karst feature, which may have acted as conduit located near the existing production well bore. Two horizontal infill wells were planned in this field with the objectives to accelerate the gas production, and to investigate the uncertainty on the current gas water contact (GWC) movement, karst feature and fracture identification in single run LWD. The horizontal well placement strategies were set in different workflows as some key reservoir parameters are still uncertain. Several scenarios were made based on real-time reservoir characterization and observation, then they were translated into different production performance profiles. To achieve those well objectives, a new technology approach was proposed that consisted of the latest reservoir mapping while drilling technology with the capability to map the reservoir structure and fluid contact within 35m depth of investigation. This was combined with the full suites of log measurement including neutron, density and sonic data for secondary porosity measurement to support the real-time petrophysical and well placement decision making. The two horizontal infill wells were planned to stay close to the top of carbonate and to avoid the water contact as much as possible in order to optimize the gas production. The second well would depend on the first well reservoir observation result. On this case study, the application of reservoir mapping technology while drilling has successfully demonstrated the well placement as close as 7m TVD below the reservoir roof and mapping the gas water contact at 30m TVD below the trajectory. The actual GWC was detected 25m shallower than prognosed, so the finding of this first well has led to the decision to eliminate the drilling of the second well, which resulted in a cost saving of more than USD 20 million. Other than that, the qualitative result from reservoir mapping tool provided a new understanding of carbonate reservoir modeling, which confirmed the interpretation, current GWC and reservoir heterogeneity characterization both vertically and laterally. This strategy could be replicated in other carbonate reservoirs to delineate current gas water contact without physically penetrating it. Karst and secondary porosity interpretation were used for completion optimization and to maximize the production.
Muhammad, F. (PCK2L) | Othman, T. R. (PCK2L) | Pujiriyanto, A. (PCK2L) | Setyadi, A. L. (PCK2L) | Ardikani, N. (Schlumberger) | Azwar, O. (Schlumberger) | Pasaribu, I. T. (Schlumberger) | Permanasari, D. (Schlumberger)
Carbonate reservoirs in East Java basin had been known for its challenging drilling conditions due to the overlying shale instability and heterogeneity. The primary carbonate formation in the field has multiple reservoir targets with different flow units, which are dominantly due to secondary porosity resulting from karst processes. These reservoirs are relatively thin with net thickness ranging from 10 to 40 ft. Thus, the amount of producible hydrocarbon will depend on the length of the drain section along the reservoir. Horizontal well was selected by the oil operator in order to have economical production rate from each of these carbonate reservoirs.
However, there are high uncertainties on the structural dips along the planned lateral length due to limited numbers of offset wells and low seismic resolution. Moreover, karst has irregular shapes and sizes that can pose additional challenges such as directional drilling control, loss circulation, and deep mud invasion that affect shallow measurements. Technology with deep directional measurements that is capable to map the reservoir boundaries and structural dipping beyond the karst zone, is therefore required to achieve the horizontal length objective. This paper highlights a successful horizontal well placement in a karstified carbonate layer utilizing the new LWD reservoir mapping technology, in conjunction with the other LWD measurements that provide the necessary petrophysical information for thorough reservoir evaluation. This new deep directional electromagnetic technology extends the depth of investigation to 100 ft or more from the wellbore and resolves multiple layers with contrasting resistivity, for reservoir-scale imaging capability.
As a result, the lateral drain section was maintained successfully in the target reservoir with total net length of ~2600-ft. This interval consists of 45% length in karst or secondary porosity and 55% in the matrix-dominant porosity. Multiple layers with depth of investigation up to 50 ft were mapped in the inversion result from Reservoir Mapping-While-Drilling technology, where top and bottom reservoir boundaries were continuously mapped during drilling. In addition, reservoir mapping application in this well revealed the various heterogeneity natures in the reservoir such as lateral changes due to karst system diagenesis process and a sinkhole feature. These informations are useful for improved reservoir evaluation and delineation.
The integration between reservoir-scale imaging and the geological concept of the field not only has the capability to map the reservoir boundaries and structural dips around the trajectory, but also led to optimum steering decision. Following the success, similar method and technology will be used in the subsequent horizontal wells in this field.