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The development of well completion strategy at the subject field has evolved from simple vertical completions to extended horizontal wells with selective openhole completions (Etuhoko et al. 2014). Normally, well completions in this field involve large volume acidizing treatments. Matrix acidizing and acid fracturing stimulation are widely used in carbonate formations to improve oil recovery arising from the high solubility of the carbonate formation in hydrochloric acid. The ability to create wormholes in the near-wellbore vicinity provides a stimulation effect by connecting to secondary porosity (natural fractures, voids, and vugs). The connection of the natural fracture network to the wellbore utilizing acid fracturing treatments provides a stimulation effect because the hydraulically fractured reservoir increases the effective wellbore radius and results in a negative skin factor. However, massive acid treatments raised certain issues after beginning to develop oil-bearing zones near water/oil contacts. Significant water production occurred in some flank wells after acid stimulation. High permeability channeling is suspected between the aquifer and the wellbore. There is a high probability of water flow through natural fracture networks because no barrier exists to prevent drainage from the aquifer, which is connected to the wellbore with acid-stimulated wormholes or via acid etched fracture faces. Because production facilities are not designed to handle a high water volume, production wells with an unfortunate water cut are shut down. Obviously, field economics suffer and an effective water shutoff solution is now a vital component of future field development.
Although many advances have been made in both mechanical and chemical methods to reduce unwanted water production, horizontal wellbores present challenges. If water breakthough is recorded, the field operator conducts a multiphase production log (MPLT) survey to identify the influx depth. The first choice for water shutoff is usually a mechanical isolation in which the water-producing interval is either shut in by a sliding sleeve (SS) or the entire wellbore is isolated by bridge plugs. Unfortunately, mechanical isolation does not help because the water flow easily bypasses the restriction by flowing through the carbonate formation and enters the wellbore in the upper interval ( Schematic of horizontal well before the chemical water shutoff treatment.
Schematic of horizontal well before the chemical water shutoff treatment.
This document describes the materials used for chemical water shutoff purposes and the history of the technology application in the subject field. Avoiding known pitfalls and following best operational practices may help to increase the success rate of chemical water shutoff treatments.
Naik, Sarvesh (Chevron Energy Technology Company) | Dean, Mark (Saudi Arabian Chevron) | McDuff, Darren (Chevron Energy Technology Company) | Ranson, Andrew (Saudi Arabian Chevron) | Jin, Xiao (Texas A&M University) | Zhu, Ding (Texas A&M University) | Hill, Alfred Dan (Texas A&M University)
Saudi Arabian Chevron (SAC) partnered with the Texas A&M University Petroleum Engineering Department and Reservoir Productivity Geomechanics Team of Chevron's Energy Technology Center (ETC) to perform acid fracturing conductivity tests on the Ratawi Limestone core samples. These tests were also performed on an analog limestone from an onshore USA field and Indiana Limestone samples for comparison with the results from the Ratawi Limestone samples. This paper shows the results of the acid fracture conductivity tests using various acid treatment systems on three different limestone formations and compares the acid etching and conductivity responses between homogeneous and heterogeneous mineralogy.
The success of acid fracturing treatment depends on the creation and sustainability of fracture conductivity under reservoir conditions. The fracture conductivity depends on the reservoir rock & acid reactivity, acid-etched pattern, closure stress on the fracture face and the pore pressure depletion.
Laboratory testing shows that acid fracturing is a viable option for large-scale development of the Ratawi Limestone. Its heterogeneous mineralogy plays an important role for sustaining the fracture conductivity after acid injection. Composed primarily of calcite and dolomite, limestone dissolves positively in acid. However, the insoluble minerals, such as the clay streaks with higher mechanical properties, acted as pillars to partially prop the fractures open as closure stress was applied. Essentially, the heterogeneous mineralogy of this formation assists with sustaining fracture conductivity as the reservoir pressure depletes.
In the oil & gas industry, surface seismic imaging is critical in defining the sub-surface structure & stratigraphy but could suffer from poor seismic resolution due to complex geology. This paper describes applications of Walkaway and Walkaround VSPs recorded over an oilfield with low surface seismic resolution located in the offshore Arabian Gulf. Two further challenges needed to be addressed: i) estimation of azimuthal anisotropy, and ii) calibration of PS seismic data.
Walkaway VSP was proposed as a viable solution to obtain the higher seismic resolution by placing the receiver below the strong shallow reflectors. A multi-azimuthal (4 orthogonal lines) walkaway VSP survey was acquired based on pre-survey modeling to achieve the objectives of higher resolution and calibration of PS data obtained during OBC acquisition. Walkaround VSP was also acquired in two settings: a deep setting to estimate the azimuthal anisotropy to provide information on presence of natural fractures in the main producing reservoirs, and a shallow setting to investigate the causes of a major lost circulation zone.
3-Component downhole receivers and a powerful seismic source (6-Gun Cluster) were used to acquire good quality signal below the near surface attenuating layers. Passing through an elaborate 3-component processing chain, the walkaway survey resulted in hi-resolution subsurface PP and PS images (both in time & depth). These images provide detailed sub-surface features which were not observed in the existing surface seismic, resulting in improved reservoir characterization, as well as calibration of the surface PS seismic data. Walkaround VSP data were interpreted in terms of fracture azimuth (Fast Shear direction) and fracture density variation with depth (ratio of transverse to radial amplitudes). The Walkaround VSP results show good correlation with core data across two discrete reservoir intervals. Walkaround VSP data recorded across the shallow lost circulation zone showed strong anisotropy that coincides precisely with the lost circulation zone indicating high fracture density, with the fast velocity azimuth being consistent with the regional stress field.
It was the first time that high resolution VSP images (vertically and spatially) were generated for this field. High resolution walkaway VSP images were utilized to enhance the existing horizon and fault interpretation and reservoir characterization. The Walkaround VSP also helped to understand the azimuthal anisotropy and presence of natural fractures near the well in the main producing reservoir.
Reservoir characterization and quantification of initial oil in place in carbonate reservoirs are challenging steps, where reliability of the produced data and its uncertainty level will be critical for reservoir development. The field, under study, is a complex carbonate reservoir, with variations in its fluid contacts and stratigraphic settings. This carbonate reservoir requires an integrated petrophysical evaluation study that involves geology, geophysics, openhole logs, special core analysis (SCAL), reservoir rock typing (RRT), saturation height model (SHM) and detailed reservoir/fluid analysis, that would result in reliable reservoir characterization, fluid contacts, and fluids in place estimation with least levels of uncertainty.
An innovative integrated petrophysical evaluation has been conducted for reservoir characterization. Using core data, reservoir quality index (RQI) bins have been imposed over porosity-permeability cross-plots to define the reservoir rock types, RRTs, in this carbonate reservoir. An advanced permeability log has been developed from porosity, clay/bound water, pore size, core, diagenetic and mobility data, have derived for each rock type. The Saturation Height Model (SHM) was based on SCAL data of Mercury Injection Capillary Pressure (MICP), capillary pressures-water saturation from Porous plate/Centrifuge and Relative Permeability for every RRT for reservoir subunits. The free water level (FWL) derived from formation pressure has been used to identify an initial oil water contact, and thus it has provided good comparison of SW_Log & SW_SHM for reservoir modeling. An initial water saturation, relative permeability and residual oil saturation from SCAL data have been used in evaluation. The log data of water saturation from resistivity, core saturation height model and reservoir pressure depletion in production wells have been evaluated for an improving reservoir characterization.
The advanced and integrated evaluation approach of subsurface and well test data has been used to provide reliable carbonate reservoir properties and results on porosity, permeability, fluid contacts, reservoir rock typing, initial water saturation, and initial oil in place. It has provided reliable reservoir characterization, reservoir modeling approach for this complex carbonate reservoir. Hence, this approach has provided reliable assurance and important benefits for reservoir characterization, optimization and reservoir management.
The integrated petrophysical approach of this paper presents an integrated reservoir petrophysical approach for carbonate reservoirs. It is an integrated approach that used the available log, core, dynamic pressure data, to derive reservoir rock types, assignment of permeability as a function of porosity, pore size, clay/bound water, diagenetic and mobility data. Integration of available petrophysical, pressure, and SCAL data to derive the reservoir fluid contacts, and develop SHMs for oil in place estimation and reservoir modeling purposes.
Bigoni, Francesco (Eni S.p.A) | Pirrone, Marco (Eni S.p.A) | Trombin, Gianluca (Eni S.p.A) | Vinci, Fabio Francesco (Eni S.p.A) | Raimondi Cominesi, Nicola (ZFOD) | Guglielmelli, Andrea (ZFOD) | Ali Hassan, Al Attwi Maher (ZFOD) | Ibrahim Uatouf, Kubbah Salma (ZFOD) | Bazzana, Michele (Eni Iraq BV) | Viviani, Enea (Eni Iraq BV)
The Mishrif Formation is one of the important carbonate reservoirs in middle, southern Iraq and throughout the Middle East. In southern Iraq, the formation provides the reservoir in oilfields such as Rumaila/West Qurna, Tuba and Zubair. The top of the Mishrif Formation is marked by a regional unconformity: a long period of emersion in Turonian (ab. 4.4 My) regionally occurred boosted by a warm humid climate, associated to heavy rainfall. In Zubair Field, within the Upper interval of Mishrif Formation, there are numerous evidences of karst features responsible of important permeability enhancements in low porosity intervals that are critical for production optimization and reservoir management purposes.
In the first phase, the integration of Multi-rate Production logging and Well Test analysis was very useful to evaluate the permeability values and to highlight the enhanced permeability (largely higher than expected Matrix permeability) intervals related to karst features; Image log analysis, on the same wells, allowed to find out a relationship between karst features and vug densities, making possible to extend the karst features identification also in wells lacking of well test and Production logging information. This approach has allowed to obtain a Karst/No Karst Supervised dataset for about 60 wells.
In the second phase different seismic and geological attributes have been considered in order to investigate possible correlations with karst features. In fact there are some parameters that show somehow a correlation with Karst and/or NoKarst wells: the Spectral Decomposition (specially 10 and 40 Hz volumes), the detection of sink-holes at top Mishrif on the Continuity Cube and its related distance, the sub-seismic Lineaments (obtained from Curvature analysis and subordinately from Continuity), distance from Top Mishrif. In the light of these results, the most meaningful parameters have been used as input data for a Neural Net Process ("Supervised Neural Network") utilizing the Supervised dataset both as a Trained dataset (70%) and as a Verification dataset (30%). A probability 3D Volume of Karst features was finally obtained; the comparison with verification dataset points out an error range around 0.2 that is to say that the rate of success of the probability Volume is about 80%.
The final outcomes of the workflow are karst probability maps that are extremely useful to guide new wells location and trajectory. Actually, two proof of concept case histories have demonstrated the reliability of this approach. The newly drilled wells, with optimized paths according to these prediction-maps, have intercepted the desired karst intervals as per the subsequent image log interpretation, which results have been very valuable in the proper perforation strategy including low porous intervals but characterized by high vuggy density (Karst features). Based on these promising results the ongoing drilling campaign has been optimized accordingly.
Identification of tidal channels fairways is key for predicting behavior of areas at higher risk to water breakthrough or otherwise have a significant impact on the development and monitoring of reservoir performance. However, tidal channels in carbonates are not often easily characterized using conventional seismic attributes. It is important to decipher the complexity of the carbonate tidal channel architecture with integrated multisource data and a variety of approaches.
In this paper, petrological characteristics and petrographic analysis is conducted on well logs and validated carefully using core data. Then, the second step is to compare the carbonate channel systems with modern analogue in Bahama tidal flat and outcrop scales in Wadi Mi'Aidin (Northern Oman). Thereafter, the supervised probabilistic neural network (PNN) and linear regression method were undertaken to detect an additional channel distribution.
The relationship of high porosity with low acoustic impedance appeared mostly in the channel facies which reflects good reservoir quality grainstone channels. Outside these channels, the rock is heavily mud filled by peritidal carbonates and characterized by a high acoustic impedance anomaly with low quality of porosity distribution. The new observation of PNN porosity volume revealed a lateral distribution of the Mishrif carbonate tidal channels in terms of paleocurrent direction and the connectivity. Additionally, the prior information from core data and the geological knowledge indicate a good consistency with classified lithology. These observations implied that Mishrif channels consist of a wide range of lithology and porotype fluctuations due to the impact of depositional environment.
The work enables us to provide a new insight into the distribution of channel bodies, and petrophysical properties with quantification of their influence on dynamic reservoir behavior of the main producing reservoir. This work will not only provide an important guidance to the development and production of this case study, however also deliver an integrated work path for the similar geological and sedimentary environment in the nearby oil fields of Southern Iraq.
Summary Chemical-enhanced oil-recovery (CEOR) mechanisms are strongly influenced by gridblock size and reservoir heterogeneity compared with conventional waterflooding (WF) simulations. In WF-simulation models, simulation grids are commonly upscaled (coarsened) on the basis of a single-phase flow to perform history matching and sensitivity analyses within affordable computational times. However, this coarse-grid resolution (typically, approximately 100 ft) is insufficient for CEOR, and hence usually fails to capture key physical mechanisms. These coarse models also tend to increase numerical dispersion, artificially increase the level of mixing, and have inadequate resolution to capture certain geological features to which EOR processes can be highly sensitive. Therefore, grid refinement is necessary for CEOR simulations when the original (fine) Earth model is not available or when major disconnects occur between the original Earth model and the history-matched coarse WF model. However, recreating the fine-scale heterogeneity without degrading the history match from the coarse grid remains a challenge. Because of the different recovery mechanisms involved in CEOR, such as miscibility and thermodynamic phase behavior, the impact of grid downscaling on CEOR simulations is not well-understood. In this work, we introduce a geostatistical downscaling method that can be conditioned to tracer data, for refining coarse historymatched WF models. The proposed downscaling method refines the coarse grid and populates the relevant properties in the newly created, finer gridblocks, reproducing the fine-scale heterogeneity while retaining the fluid material balance.
ABSTRACT: Zubair Formation is one of the richest petroleum systems in Southern Iraq. This formation is composed mainly of sandstones interbedded with shale sequences, with minor streaks of limestone and siltstone. Borehole collapse is one of the most critical challenges that continuously appear in drilling and production operations. Problems associated with borehole collapse, such as tight hole while tripping, stuck pipe and logging tools, hole enlargement, poor log quality, and poor primary cement jobs, are the cause of the majority of the nonproductive time (NPT) in the Zubair reservoir developments. Several studies released models predicting the onset of borehole collapse and the amount of enlargement of the wellbore cross-section. However, assumptions involved in these models have limited their applications to very specific scenarios. In this study, an analytical solution for determination of the volume of collapse during drilling is presented based on changes in the geometry of boreholes. The approach can be used in conjunction with different failure criteria to estimate the collapse onset in the presence of formation strengths and principal stresses. The study highlighted how the integration of information from different sources and disciplines were able to estimate the shear failure induced on the borehole wall during drilling.
During the drilling development in the Zubair Formation, shear failure (collapse) is the main instability problems. Historically, over 90% of wellbore problems in the Zubair Formation occur due to borehole collapse . These borehole collapse problems, such as tight hole while tripping, stuck logging tools, and subsequent fishing, stuck pipe, and sidetracking are time-consuming and expensive. Many times, due to a severe stuck pipe problem and unsuccessful fishing operations, the well has to be drilled with more than one sidetrack or, in the worst case, the wellbore has to be abandoned . Another difficult scenario is when the higher operational costs that are caused by lower drilling performance or more complex operations are taken into consideration. Unfortunately, this has placed constraints on field development plans .
The objective of this work is to characterize the fault system and its impact on Mishrif reservoir capacity in the West Quran oil field. Determination and modelling of these faults are crucial to evaluate and understanding fluid flow of both oil and water injection in terms of distribution and the movement. In addition to define the structure away from the well control and understanding the evolution of West Qurna arch over geologic time.
In order to achieve the aim of the work and the structural analysis, a step wise approach was undertaken. Primarily, intensive seismic interpretation and building of structure maps were carried out across the high resolution of 3D-seismic survey with focusing on the main producing Mishrif reservoir of the field. Also, seismic attributes volumes provided a good information about the distribution and geometry of faults in Mishrif reservoir. The next step, it constructs 3-D fault model which will be later merged into the developed 3D geological model. West Qurna/1 oil field situated within the Zubair Subzone, and it is structurally a part of large anticline towards the north. The observation of seismically derived faults near Mishrif reservoir indicated en-echelon faults which refer to strike-slip tectonics along with extensional faults. The statistic of Mishrif interval faulting indicates a big number faults striking north-south along western wedge of anticline. The seismic interpretation, in combination with seismic attributes volumes, deliver a valuable structural framework which in turns used to build a better geological model.
In this paper, the work demonstrates a better understanding for the perspectives on the seismic characterization of the structural framework in the Mishrif reservoir, and also for similar heterogeneous carbonate reservoirs. Further, this work will ultimately lead to improve reservoir management practises in terms of production performance and water flooding plan.
Al-Hameedi, Abo Taleb T. (Missouri University of Science and Technology) | Alkinani, Husam H. (Missouri University of Science and Technology) | Dunn-Norman, Shari (Missouri University of Science and Technology) | Flori, Ralph E. (Missouri University of Science and Technology) | Alsaba, Mortadha T. (Australian College of Kuwait) | Amer, Ahmed S. (Newpark Technology Center/ Newpark Drilling Fluids) | Al-Bazzaz, Waleed H. (Kuwait Institute for Scientific Research)
The rate of penetration (ROP) plays a major role in reducing drilling costs, making it an important area of investigation. There are various controllable and uncontrollable factors that affect the ROP, and the variation in these variables affecting the ROP made it a very pivotal drilling parameter that has a significant effect on non-productive time. In this work, sensitivity and statistical analysis were carried out using data from over 1000 wells in Basra oil fields, Iraq. The scope of this work is to determine the effect of rheological properties on ROP, to provide a method for estimating the recommended range for drilling fluid properties based on data mining techniques.
In this work, huge real field data from over 1000 wells drilled in Basra oil fields, Iraq were gathered and analyzed to better understand the characteristics of a drilling fluid that enhance ROP and quantify the impact of each drilling fluid rheological properties on ROP. The data used in this study were collected from mud logging data, daily drilling reports (DDR), and geological information. Statistical and sensitivity analyses were performed in order to identify the relationship between ROP and drilling fluid rheological properties. The correlation coefficient (CC) was utilized to understand the effect of solid content (SC), yield point (Yp), and plastic viscosity (PV) on ROP. The results showed that SC is the most influential rheological property on ROP, then PV and finally Yp. In addition, this work demonstrates how bit hydraulics can be improved by means of modifying the rheological properties rather than adjusting the flow rate or nozzle size.
Large-scale collection and interpretation of field data or in other words "data mining" can be considered as a strong tool in understanding the impact of different parameters on the ROP in order to estimate the recommended range of rheological properties, which will result in improving the ROP.