For pressure maintenance purpose peripheral wells have been used to inject sea water into a carbonate reservoir offshore Abu Dhabi. The injected water preferentially follows the path of the of higher permeability zones, since injection is done into formation water below oil water contact. Though the sea water front movement in the reservoir has been estimated indirectly via numerical reservoir simulator, successes of direct methods have been limited by the injection volume and environmental effects.Direct spatial measurement of the injected sea water front within the reservoir is important to evaluate the efficiency of pressure maintenance by peripheral water injection, also considered an important step in tuning simulator parameters, and optimizing the Field Development Plan (FDP).Although there is strong water salinity contrast between the injected and original reservoir water in this field, resistivity-based methods can be affected by variations in the reservoir rock cementation factor, while cased hole logs can be affected by environmental effects such as hydrochloric acid effect commonly seen in carbonate reservoirs after stimulation.This technique that utilizes open hole Pulsed Neutron Sigma measurement of Logging While Drilling (LWD) enables petrophysicists to distinguish and determine injected water separately from formation water, independent of Archie-based resistivity method. Also this technique provides a new methodology to calculate cementation factor (m). Successful application of this technique in an offshore Abu Dhabi carbonate reservoir is presented here.

Quantifying gas saturation in carbonate formations through cased logging remains an elusive objective. Unsolved gas saturation quantification in cased hole was always thought to be due to unknown or uncontrolled borehole completion, casing, cement, acidizing, perforation, invasion and/or formation damage. Several techniques have been developed in the past to correct for the cased-hole thermal neutrons logs in an attempt to normalize the data through laboratory experiments, correction charts or open hole computer processing of effective porosity (PHIE). In Abu Dhabi offshore lower Cretaceous reservoirs, joint petrophysical & reservoir engineering efforts have shown that most of these automated corrections and/or normalizations are not capturing the real environmental changes from open-hole (OH) to cased-hole (CH). The reasons behind it are:Open-hole neutron logs were not considered as the reference; instead, old technique uses PHIE (Effective Porosity).Casing and cement corrections are based on a homogenous isotropic model.There was no proper resolution matching between OH and CH and edge effect corrections applied to pulse neutron data. Therefore, new technique is developed and presented in this paper; it is called "HYDGO" (Hydrocarbon Density Determination for Gas and Oil). This technique utilizes several back-to-back OH and CH log data information to carefully eliminate the CH environment effect. This is done by developing multi-variable-correction model that incorporates OH bulk density, water saturation, thermal neutron porosity and invasion. This procedure has led to the development of hydrocarbon density and gas saturation determination. Introduction Gas saturation is one of the vital requirements in reservoir management due to subsequent changes in reservoir fluid saturation and characterization after starting gas injection. In general, Gas injection is important for:Maintaining reservoir pressure.Enhancing recovery to reach as closest as possible to residual oil saturation.Changing oil properties with miscible gas injection to achieve better recovery Therefore to assure that gas injection project can achieve its maximum value, HYDGO technique has been developed to:Determine gas saturation (Sg).Utilize pulse neutron logs data for purpose, as it is the only possible formation evaluation logs can go through tubing. HYDGO technique was developed and tested in Gas Injection Pilot Project in one of major offshore field in Abu Dhabi. The G-I-P-P consists of four wells, two horizontal injector wells drilled into two different subzones, one zone is designated to be a secondary gas flooding zone and the other is tertiary gas flooding zone. Also one observer and one producer are also drilled 500 meter far away from injectors. Several logs were programmed and run in this project, for both OH and CH sections. Full suite of OH logs was run to establish a baseline saturation comparison to be used later with CH logs. The OH program consists of:Induction and Laterolog ResistivityThermal and Epithermal Neutron PorosityDensity with PE (Photo Electric factor)

Asphaltene study is now becoming a regular menu as a part of gas injection studies 1–11. The asphaltene onset pressure (AOP) is one of the most important factors to understand asphaltene precipitating behavior. The SDS (solid detection system) based on light scattering technique has been quite popular and widely used in all over the world 1,7–9,12–15. The simple experiments to measure AOP are usually conducted using mixture of reservoir fluid and injection gas, and various gas mixing volume are assumed to be investigated. These various experimental specification of gas mixing volume are useful to understand asphaltene risks during gas injection projects. However, what this investigation can show is just a static asphaltene behavior, and sometimes might overlook true asphaltene risks. In the gas injection pilot (GIP) project in an offshore carbonate oil field in the Arabian Gulf, the static asphaltene behavior was studied by the SDS using NIR (neear infrared) light scattering technique. For this study, a single phase bottomhole sample was collected from the same producing zone, but the sampling location was 90 ft shallower than the GIP area. Various combination of mixtures (sampled reservoir fluid mixed with 0, 25, 37.5, 43.5 and 50 mol% injection gas) were examined to measure AOP. Furthermore, the numerical models were generated and calibrated with the experimental findings. In order to evaluate the asphaltene risks at the GIP area, the models were adjusted to the target oil composition by considering existing oil compositional gradient in the field. However, the modeling analyses showed that the operating conditions of producing wells are outside the estimated asphaltene precipitation envelope (APE). This result was inconsistent with the field fact, in which actual asphaltene deposits were observed and collected from bottomhole of some wells in the GIP area. Namely, we were obliged to judge that our current experimental results of static asphaltene behavior overlooked at the actual asphaltene risks. What is insufficient for a realistic modeling ? Our hypothesis is the dynamic asphaltene behavior. During gas injection process, the injected gas composition is changed due to vaporizing gas drive (VGD) mechanism, in which gas was enriched with the intermediate molecular weight hydrocarbons from reservoir oil. Our latest experimental investigation of static asphaltene behavior did not include this process. Therefore, the sensitivity analyses of the VGD effects were carried out with the calibrated model to realistically evaluate the actual APE. Various enriched gas composition were assumed, and the affects of these enriched gas on APE were investigated. Consequently, it was found that the enrichment of intermediate components expanded APE, and the operating condition of asphaltene problematic wells could be explained to be inside APE. Therefore, we concluded that the dynamic asphaltene behavior must be understood for a realistic risk evaluation in the gas injection project. Introduction Background and Histories The target field was discovered in 1963 and started production in 1967. It is currently operated by ADMA-OPCO. It produces from two carbonate reservoirs (A and B) and its oil is transported and processed at the plant near an island. To maintain reservoir pressure, the dump flood water injection started in 1972, followed by powered water injection in 1978 and the crestal gas injection in 2003. In addition to this project, gas injection pilot project at the flank area has been carried out at western flank area of the field.

The great Paleozoic system of the Arabian Gulf forms one of the most prolific hydrocarbon producing systems in the world. The Primary elements of this system like source, reservoir, seal and traps are extended throughout the eastern Arabian Peninsula and have exceptional scale and quality. Modeling of the structural growth history, in conjunction with the hydrocarbon generation and migration in the area, has shown that early growth and charge were essential conditions for the formation of economic hydrocarbon accumulations.

This paper covers the integrated approach performed in one of the major oil fields in off-shore Abu Dhabi to increase the confidence level in this tight heterogeneous reservoir development.This paper outlines the challenges that were faced along with the history of this field and the different approaches made by the development team to overcome them. The paper discusses the steps that the team went through in a period of ten weeks in order to come up in the end with an optimized subsurface & completion strategy plan. It shows how the reservoir with three main reservoir subzones is divided into 43 layers based on the sequence stratigraphy approach. It also discusses how the new infill well locations were optimized and designed with a fit-for-purpose completion strategy. The paper will present the final outcomes of this review, the opportunities identified and the latest results obtained from the more confident post optimized development plan.