The Barik, Miqrat and Amin formations are deep, tight reservoirs of the Haima Supergroup that provide the majority of gas production in the Sultanate of Oman. The Miqrat formation is a feldspathic sand/shale sequence with complex pore structure and occasional bitumen presence. In the area of interest, it occurs at a depth of approximately 5000 m. Average porosity varies from 5 to 9%, average permeability for Lower Miqrat does not exceed 0.1 mD. In general, Archie equation derived saturation in low porosity rocks is subject to medium to high uncertainty. Therefore the most common challenge in the petrophysical evaluation of tight reservoirs is the determination of gas saturation and fluid type identification.
In an effort to improve the reliability of saturation calculation and fluid typing, several different methods were tested including cased-hole Pulsed Neutron Logs (PNL). The classical sigma interpretation was found to be too sensitive to input parameters and did not provide significant improvement to saturation determination in the complex Haima lithologies. An important breakthrough was made when the dynamics of the mud filtrate invasion process in these reservoirs was understood. During open-hole logging usually very little or no gas effect is observed on logs with negligible or no density-neutron separation. The reason is considered to be deep mud filtrate invasion pushing moveable gas beyond the depth of investigation of radioactive logs. One or two months later, the filtrate in the invasion zone dissipates with gas returning to the near wellbore formation.
The best match between log calculated moveable gas saturation and production test data was obtained using a reverse time-lapse technique, with PNL cased-hole logs compared to baseline open-hole neutron measurements. The changes in neutron porosity with time can be attributed to moveable gas saturation. Careful neutron log quality control and normalization across non-reservoir and known water-bearing sections is required. Knowing the hydrogen index of gas, we can calculate the moveable gas saturation from the difference in neutron log response. In contrast to the sigma approach, an accurate rock matrix model is not required.
This paper describes the Reverse Time-Lapse technique: a novel application of the classic time-lapse technique between open-hole neutron and cased-hole PNL. The case studies demonstrate that this technique is applicable for completion decision making and field-scale development planning.
AL Isaee, Omar (Petroleum Development Oman) | Chavez Florez, Juan (Petroleum Development Oman) | Ali, Nada (Schlumberger) | AL Ghatrifi, Rawan (Petroleum Development Oman) | Al-Yaqoubi, Mazin (Petroleum Development Oman) | AL Abri, Ahmed (Petroleum Development Oman) | AL Hinai, Mohamed (Petroleum Development Oman)
In Oman, the unique geological properties of the reservoirs require different fracture strategies and technology deployment to make them commercially viable. Highly deviated wells, with multiple hydraulic fractures, have been identified as key technology enabler for the development of tight gas accumulations in Oman. The main objective of this study is to generate a 3D petrophysical and geomechanical view of the reservoir, to have a better understating of Hydraulic Fracturing for Horizontal and Highly Deviated Wells The comprehensive amount of data captured during the initial implementation phase of highly deviated wells covering reservoir characterization, fracture geomechanics as well as production logs in combination with the existent data captured in vertical wells, proves to be complex to analyze due to the volume of information and the multi variable nature associated with fracture and inflow predictions. A methodology was required where correlations and tendencies were identifiable at structural level, covering all target gas accumulations using all the static and dynamic captured data. The definition of a 3D Grid Visualization Block (3D-GVB) was introduced where all the captured parameters were distributed for analysis and interpretation. As a result of the appraisal and initial field development with vertical wells, it was possible to identify tight accumulations that will require dedicated highly deviated wells for its development. The initial phase of the implementation of highly deviated wells proves to be challenging, as the observed heterogeneities on geomechanical and petrophysical properties across the target gas accumulations, combined with differential depletion and the wells orientation to generate transverse fractures, creates a complex environment for fracture initiation and propagation, impacting not only fracture deployment but inflow deliverability of this wells. This paper will describe how the methodology uses a cycle of data analysis and interpretation to identify tendencies, that will lead to correlation and new algorithms that are retrofitted on the 3D-GVB platform, leading to optimization of well positioning at structural level, drilling and completion of this highly deviated wells. It will be described how this methodology is used for well positioning at structural level, to define well architectures oriented to enhance not only drilling, but also hydraulic fracturing and hydrocarbon deliverability on highly deviated wells. 2 SPE-197901-MS
Ibrahim, Ehab (Petroleum Development Oman) | Sayapov, Ernest (Petroleum Development Oman) | Hinai, Rashid (Petroleum Development Oman) | Qarni, Sulaiman (Petroleum Development Oman) | Kristanto, Royke (Petroleum Development Oman)
In low-permeability formations such as tight gas reservoirs, a well would be economic only if an effective hydraulic fracturing technique is selected. In central part of Sultanate of Oman a deep tight gas field is developed with hydraulic fracture stimulation. Normally, between 7 and 13 frac stages are done per well. Majority of wells are vertical with pay zones separated with shale layers that prevent fracture growth. Plug & perf is a common technique used in this field, therefore there are multiple well interventions during Hydraulic Fracture operations that consume time and delay the well delivery. By deploying multistage frac completion with the objective of producing, enhancing and cost/time savings, the effectiveness of the fracturing operations was expected to increase. Equipment selection, design and development was performed based on well conditions, casing design, operational parameters and production gas composition.
Multistage frac completion allows the frac operation to be continuously performed without the need to conduct intervention activities such as running/setting frac plugs, perforating, milling and clean-out between intervals. The intervention activities can be conducted at the end of the frac operation in single-trip deployment if desired. The success in North America in horizontal tight gas wells has opened a door for implementation of this system in vertical wells in Sultanate of Oman. The main challenge in deployment of this system in vertical wells is the accurate positioning of the sleeves. The shale layers between the pay zones could be as narrow as 5 m or less and a small pay zone might be easily missed. Besides, deployment and cementing operations are equally essential as proper zonal isolation is a must with water zones embedded in between.
This paper is discussing the lessons learned from utilization of multistage frac completion in vertical deep wells (around 5000 m) covering the completion and Hydraulic fracturing stimulation operations. This technique has proven significant cost & time reduction and production increase as well as reduced HSE exposure contributing to better gas recovery, improvement in operator's performance and energy delivery to the country.
Sayapov, Ernest (Petroleum Development Oman) | Al Farei, Ibrahim (Petroleum Development Oman) | Al Salmi, Masoud (Petroleum Development Oman) | Nunez, Alvaro (Petroleum Development Oman) | Al Shanfari, Abdulaziz (Petroleum Development Oman) | Al Gheilani, Hamdan (Petroleum Development Oman) | Smith, Andy (Welltec) | Yakovlev, Timofey (Welltec)
In recent years, horizontal drilling has become increasingly important to the oil and gas industry to enable efficient access to complex structures and marginal fields and to increase the reservoir contact area. New technologies have emerged during this time to address post-drilling intervention challenges in such wells. However, complexity of operations in horizontal wells is much higher than that of the vertical wells; therefore effectiveness of the selected technique has a major impact on the operational success and economics. In depressed market environment, economical and operational effectiveness becomes even more important especially when it’s down to complicated, challenging projects that require not only large investments but also simultaneous and continuous utilization of multiple resources, technical disciplines and assets. This paper reviews and compares different ways of horizontal multizonal well preparation for hydraulic fracture stimulation using plug & perf technique in challenging downhole conditions - differential pressures over 15,000 psi, presence of depleted zones complicating cleanout and milling operations between the frac stages, depth control issues.
In PDO, there are some gas fields sharing similar downhole conditions whereas fracturing operations are complicated by the requirement of CT cleanouts and/or milling in between the stages. A horizontal well development trial has been implemented to evaluate its economic efficiency and prospects. Depending on the success of this trial, this approach can be spread to other fields with similar characteristics. In these trial wells, multistage completion technologies were not available due to either differential pressure limitations, downhole conditions or completion restrictions, therefore conventional plug & perf approach had to be applied. Such approach, in turn, becomes very challenging in horizontal wells crossing several different formations having multiple severely depleted intervals along the wellbore. These challenges include not only cleanout efficiency and precise depth control during zonal isolation and perforation but also conveyance capabilities.
Several different techniques have been tried in PDO so as to discover the most efficient and economical way to complete this task: CT with deployed wireline cable, CT with fiber optic cable, DH tractors and conventional CT with GR-CCl tools in memory mode. All of them have their pros and cons and while saving some money in one small thing, a technique may cause major losses in the other and an operator needs to select the optimum approach taking into consideration multiple aspects.
All technologies covered in the paper are well known in the oil business; however some of them were tried in an uncommon environment. For example, although not commonly used in horizontal frac applications (except for perforating for the first stage), tractors were used for plug setting and perforating between the stages and that required well cleaned wellbore for each run which is not an easily achievable task in a horizontal wells with multiple depleted zones. With certain measures aimed to improve their performance, tractors proved their efficiency; these measures are also discussed in this paper. Advantages and disadvantages of CT conveyance in comparison to tractor have also been discussed.
E-line tractor technology has been successfully deployed in the Sultanate of Oman for reservoir surveillance using production logging assemblies in mature fields. Tractors provide specific advantages, as compared to other forms of conveyance, such as coiled tubing, and can successfully negotiate complex well trajectories in both horizontal openhole and cased hole well completions, enabling acquisition of good quality flow profiles in producers and injectors.
BP is developing the Khazzan and Ghazeer fields of Block 61 in the Sultanate of Oman. The development includes three Cambro-Ordovician tight gas sand reservoirs which require hydraulic fracturing for commercial production rates. There are challenges with depth and high temperature for the open hole logging environment, with a restrictive inner diameter and residual proppant creating challenges for the cased hole logging environment. Additionally, there are cost challenges on all data acquisition including coring, downhole gauges, sampling, proppant tracers and many other forms of surveillance.
This paper outlines the evolution of the data acquisition strategy for the Khazzan and Ghazeer assets. The development plan at project sanction was 20 vertical and 272 horizontal wells. The data acquisition strategy led to the development of a data acquisition plan, and all stakeholders were engaged to ensure the right data was acquired in the right place at the right time. Cross functional behaviours and fiscal discipline were essential in this process. Inclusion of the service companies into the wider BP team was crucial to ensure appropriate technology was applied, learning from previous operations implemented and new technology options made available.
Through careful management of the data acquisition plan, all data in development wells prior to first gas were acquired within the allocated data acquisition budget despite drilling 20% more wells than originally planned for this period. Early improvement in subsurface understanding enabled an overall reduction in well count for the life of the project, extension of the original development into unpenetrated areas, adding significant value to the project.
Miqrat is a complex clastic deep tight gas reservoir in the North of the Sultanate of Oman. The Lower unit of the Miqrat formation is feldspatic sand characterized by low permeability not exceeding 0.1 mD and porosity up to 12 %. Based on results of the appraisal campaign of Field X, it contains significant volume of gas. However the production test data after fraccing showed mixed results. The objective of this study to explain the production behavior in relation to the frac geometry.
Understanding the reason of possible overestimation of log derived Hydrocarbon saturation is important. Thus the interpretation of conventional and special logs was revisited. In parallel, all the available core data including SCAL and thin sections were dissected. Besides, the analysis of hydraulic fracture propagation, well tests, cement quality, PLT including Spectral Noise Log was performed.
The wells were subdivided into categories according to their production. wells producing no water wells with water channeling from the water leg of Middle Miqrat wells with transition zone intervals with two-phase inflow of water and gas.
wells producing no water
wells with water channeling from the water leg of Middle Miqrat
wells with transition zone intervals with two-phase inflow of water and gas.
There are three main challenges that needed to be overcome. First challenge is to identify the high uncertainty in hydrocarbon saturation from the resistivity logs. Petrophysical evaluation shows that porosity profile derived from logs looks very similar in all wells with insignificant lateral variations. Hydrocarbon saturation estimated from logs looks also similar regardless of how deep or shallow the well is. However, production tests show different results, e.g. different flow rates and high water-cut are observed in some wells.
The second challenge to keep the frac height below the boundary between Lower Miqrat and Middle Miqrat, which consist of around 3 to 7 meters of shale and in most of the field it is bound with water. The third one is to cover the upper part of the zone below the shale since it is the best part of Lower Miqrat without breaking to the water leg of Middle Miqrat. A geomechanical model was created and several frac model iterations were run since in the early appraisal well that boundary was broken.
Investigation through multidisciplinary integrated team led to unlock the tight gas reserves in Lower Miqrat. Based on open hole log interpretation to create a geomechanical model. That model is being calibrated with DFIT, 3 different case hole logs and confirmed with production.
Miqrat is a complex clastic deep tight gas reservoir in the North of the Sultanate of Oman. The Lower unit of the Miqrat formation is feldspatic sand characterized by low permeability not exceeding 0.1 mD and porosity up to 12 %. Based on results of the appraisal campaign of Field X, it contains significant volume of gas. However the production test data after fraccing showed mixed results. The objective of this study was to explain the production behavior and identify the sweetspot area for further development.
Understanding the reason of possible overestimation of log derived Hydrocarbon saturation is important. Thus the interpretation of conventional and special logs was revisited. In parallel, all the available core data including SCAL and thin sections were scrutinized. Besides, the analysis of hydraulic fracture propagation, well tests, cement quality, PLT including Spectral Noise Log was performed. The wells were subdivided into categories according to their production results: wells producing no water wells with water channeling from the water leg of Middle Miqrat wells with transition zone intervals with 2 phase inflow of water and gas.
wells producing no water
wells with water channeling from the water leg of Middle Miqrat
wells with transition zone intervals with 2 phase inflow of water and gas.
Based on the integrated analysis, extend of the gas and transition zones was established, and the location of future wells optimized. From the Spectral Noise log and Temperature data, water crossflow/channeling from Middle Miqrat was identified in 2 wells, either because of broken thin sealing shale above Lower Miqrat or due to poor cement quality. The sweetspot area with commercial production was mapped. Substantial gas volumes have been unlocked. Besides, an explanation of the uncertainty in log derived saturation was suggested. Core plugs and thin sections revealed presence of partially filled vugs, which is not a typical case in a clastic environment. The origin of this porosity is puzzling and likely due to dissolution of early diagenetic nodules. The rock with poorly connected vugs has high resistivity even if it is water bearing. The review of capillary pressure data revealed that the transition zone could exceed 100 m. This finding is consistent with the interpretation from well tests.
The most practical implication of the current study potential of Lower Miqrat is unlocked. The integration of Open hole and cased hole logs and the additive information from Spectral Noise log for channeling/crossflow identification is shown. Presence of vuggy-like porosity in clastic sections and the impact of isolated vugs on log derived Saturation is demonstrated.
Core-calibrated petrophysical rock typing for gas rate deliverability profiling, coupled with field-calibrated mechanical stress models, represents a significant step forward in optimizing the value of hydraulic fracture stimulation in the Khazzan field unconventional tight gas reservoir. Our Mechanical Earth Model (MEM) is an important component of the integrated workflow currently being used for selecting intervals for successful hydraulic fracture initiation. When incorporated into an integrated subsurface performance prediction strategy, this technology enables optimization of well targeting, increased reserve recovery, and capital efficiency.
The interaction of regional tectonics and local lithology controls the stress profiles of the layers in the Barik tight gas reservoir in the Khazzan field. Core measurements show a significant effect of rock fabric and diagenesis on effective gas permeability, elastic properties, and rock strength. The ability to incorporate lithology, rock fabric, and pore geometry in our dynamic to static calibrations provides new insights into our predictions of rock mechanical properties and reservoir quality, which are used to obtain calibrated "fracture initiation" profiles from core, log, and downhole stress measurement information.
The MEM is calibrated using an integrated wellbore stability analysis and horizontal stresses are refined by honoring the observed borehole breakouts, formation breakdown and closure pressures. The models show that the adjacent layers in Khazzan field are under higher stress than the pay zones, which enhance fracture height containment and lateral fracture penetration. Previous studies have suggested that there are unequal horizontal stresses which are potentially due to tectonic effects. However, in some cases the magnitude of stress variation and reversal in stress-ordering across minor depths is problematic and confirms the pitfalls of oversimplified assumptions and models used in stress profiling in unconventional reservoirs. This work highlights the magnitude of stress variations within the formation and illustrates an integrated methodology to assist the decisions on selecting hydraulic fracture locations.
Identifications of gas in tight clastic reservoirs of Oman considered of high value as this would improve significantly well and reservoir management of a number of tight gas fields by finding more re-perforation opportunities, identifying water shut off opportunities and in-fill opportunities, and by optimizing gas production system. As a result, a study was conducted to see if PNC logs could be used for gas identifications in tight clastic reservoirs of Oman. Application of PNC over the tight gas environments pose two major challenges: 1. Consistent environmental corrections of PNC logs in the gas filled boreholes and 2. Identification of water free gas bearing zones from water bearing and gas-water transition zones These limited the use of the PNC tools in the tight gas reservoirs in Oman until PNC logs were recently used for well and reservoir management of a number of major tight gas reservoirs in PDO. This paper presents an integrated collaborative approach between operator and service provider that enables such use.
The petrophysical evaluation of tight gas formations has traditionally been centered on calculations of porosity and water saturation. These two parameters are used to quantify the original volumes in place but they do not provide information about phase mobility except at the saturation endpoints in high porosities. Low porosity affects the accuracy of water saturation calculations and can often make them ambiguous, leading to wrong decisions and unwanted water production.
We found dielectric dispersion logging to be a robust technique for determining gas pay zones independently from saturation equation input parameters. Dispersion analysis of the conductivity and permittivity measurements acquired by these tools is a function of the water tortuosity factor (mn). This factor is vitally important for accurate water saturation evaluation, but is often unknown or variable.
Nuclear magnetic resonance (NMR) measurement has the potential to enhance traditional formation evaluation techniques by providing estimation of the irreducible water saturation (Swirr) and permeability throughout the interval of interest. Accurate determination of these parameters benefits the selection of perforation intervals and improves the chances of producing maximum hydrocarbon with minimum water. NMR logging of deep tight gas formations poses unique challenges with regards to data acquisition due to low porosity, high temperature, and frequently saline muds. Pulse sequences and quality control procedures are used to validate the NMR measurements at high temperatures and high salinities.
An interpretation workflow was developed to integrate dielectric dispersion and NMR data and the results compared with more traditional formation evaluation techniques. There were significant improvements in the prediction of hydrocarbon- and water-producing intervals. The technique has been applied in several deep, high-temperature, low-porosity gas wells. These analyses are made in a timely fashion to provide operators with information for making better completion decisions.