F. Boukadi

A Comparative Study of Lost Circulation Materials

In this article, the objective is to identify an alternative lost circulation material (LCM) to combat typical operational problems associated with drilling mud losses. These problems cost the local oil company significant downtime. In the oil industry, and, of course, to the local oil company, most of the commercial lost circulation materials have been tested with different levels of success. The trend now is to rethink different ways to better tackle the mud losses intricate problem and reduce drilling costs. The objective is to investigate possible gelling and bridging agents that are currently used by other industries. To start, an extensive literature search for possible LCM candidates from industries like construction, refrigeration, pleasure-boat manufacturing and others is embarked on. The next challenge would be to envisage a thorough chemical screening and testing program to come up with an LCM that could be effective if pumped through thief zones. The effectiveness yardstick could then be measured by: (1) solving the problems of placement, (2) temperature, (3) pressure, etc.

PVT EMPIRICAL MODELS FOR SATURATED OMANI CRUDE OILS

ABSTRACT Pressure–volume–temperature (PVT) properties are critical to reservoir as well as production engineers, in particular. PVT properties could be determined experimentally. But, experiments are time consuming and costly. Moreover, laboratory PVT analysis does not consider the variations of PVT properties with respect to temperature since they are measured at reservoir temperature at the time of sampling. For that matter, the data is not benefitable. But, even if experimental analysis is done, it is difficult to obtain representative results to develop a new field. To tackle the above and other related problems, relying on sound PVT emperical correlations would be the ultimate solution. In this work, the intent is to develop stochastic models for PVT properties pertaining to Omani crude oils since it is believed that such correlations are scarce and not very precise. The empirical equations are developed for saturated Omani crude oils. The correlations are tested and validated. The empirical equations evaluation and assessment are done against existent experimental data and published correlations.

ESTABLISHING PVT CORRELATIONS FOR OMANI OILS

An adequate knowledge of any reservoir fluid PVT properties is essential for most types of petroleum calculations. These calculations include amount of oil in the reservoir, production capacity, variations in produced gas-oil ratio during the reservoirs production life, calculation of recovery efficiency, reservoir performance, production operations and the design of production facilities. PVT properties can be measured experimentally by using collected bottom-hole or surface samples of crude oils. But, the experimental determination of PVT is time consuming and very costly. In addition, even with the availability of PVT analyses, it is often necessary to extrapolate the data to field and/or surface conditions through the use of empirical correlations. Furthermore, geological and geographical conditions are considered very critical in the development of any correlation. But, universal correlations are difficult to develop. That is why correlations for local regions, where crude properties are expected to be uniform, is a reasonable alternative. In this study, experimental PVT data for North and South Oman crudes, statistical and artificial neural network (ANN) analyses are used to develop reliable PVT correlations. Comparisons with previously published correlations are presented.

EFFECT OF PVT PROPERTIES VARIATION WITH DEPTH IN HYDROCARBON RESERVOIRS

Gravity segregation in thick reservoirs, in particular, affects the distribution of hydrocarbon components in a fluid column. Given sufficient time, heavy molecules will migrate towards the bottom of the hydrocarbon column to form a mat of viscous oil. Compositional grading can be in the origin of miscalculation of reserves. In fields producing volatile oils with heavy ends, in particular, the oil formation volume factor (FVF) can vary significantly with depth. That can lead to overestimation or underestimation of the reserves depending upon fluid sampling depth. For that reason, subsurface sampling depth is critical and fluid sampling from different depths only can be considered as representative sampling. In this paper, assessment of compositional grading was explored using variation of reservoir fluid properties with depth. Different hydrocarbon column thickness have been tested to prove that grading will be enhanced for reservoir thickness exceeding 164 feet.

Evaluation of Polyurethanes as Potential Mud Loss Control Agents

In this article, polyurethanes have been selected as potential lost circulation materials (LCMs) that will be introduced in wellbores having experienced serious drilling mud losses. The chemicals will be mixed at surface, pumped downhole, let to bridge/foam and seal open fractures/channels. Different tests have been carried out to investigate better ways of mixing and handling polyurethanes before pumping. In that, prospects of slowing down polyurethane reactions by decreasing the amount of activator and adding different proportions of diesel were investigated. Testing the effect of moisture on polyurethanes also was tested. Furthermore, testing using diesel, toluene, xylene, Musol, acetone, glycol, light crude oil, Safrasol as carrier fluids was undertaken. The mixing sequence of diluents and polyurethanes also was thought of and the effect of mixing water on foaming reaction also was investigated. Placement under pressure of pumped polyurethanes also was investigated. It was revealed that pressure enhances polyurethane sealing capability. Adhesion and sealability experiments, in open fractures with apertures of 0.5 mm and 1.5 mm, also indicated that tested polyurethanes are the best candidates guaranteeing a good spreadability within open fractures. Bridging and sealing fracture apertures of up to 5 mm have been achieved with success. Sealability of even larger fracture apertures amounting to 2 inches also has been accomplished.

Pressure–Volume–Temperature Empirical Formulations for Undersaturated Omani Oils

Abstract Pressure–Volume–Temperature (PVT) empirical correlations result from analysis of experimental data collected from laboratory measurements of phase behavior properties of hydrocarbon systems. The correlations are only applicable for the tested hydrocarbons. They could nevertheless be used to predict PVT properties of similar hydrocarbons with pressures and temperatures falling within the measurements’ data range. The developed correlations are mostly, local, some are “region-bound” and a few are global. Regional and global correlations are used with caution. The reason being that hydrocarbon systems from different regions have different composition and paraffinicity, regardless of comparable temperature and pressure ranges. As a result and due to a complex geology resullting from unique depositional environments of most Omani reservoirs, a local correlation to derive PVT properties for Omani crude oils is needed. The correlations could also be used to supplement PVT experiments often time consuming and extremely costly. In this paper, the developed correlations are derived for undersaturated Omani black-oil.

A Parametric Equation Relating Resistivity to Capillary Pressure for Omani Sandstone

Abstract Capillary pressure and resistivity are important parameters that are mostly used in reservoir and petrophysical characterization. Capillary pressure data remain difficult to obtain and classical experimental methods used are questionable and not very adequate. One major drawback in running capillary pressure experiments is the use of fluids other than formation fluids. Another resides in the difficulty to measure capillary pressure at representative reservoir conditions. One more drawback, to name a few, is the experiments time which remains very prolonged, especially when the formation to be characterized is tight because the fluid drainage can take up months if not more. To overcome the intrinsic difficulties of having representative data, an empirical equation relating resistivity to capillary pressure was developed. In this work, twenty-five core plugs of Omani sandstone have been used in an attempt to come up with a representative correlation.

Selective completion for horizontal wells drilled in a fractured omani reservoir

ABSTRACT Reservoir simulation sensitivity analysis and unit technical cost analyses are used in order to determine the optimum well performance of a horizontal well that is selectively completed in one of the fractured Omani reservoirs. A black-oil model representing one heavily fractured Omani field was used in the analysis with a match box-like reservoir structure. The fractures are taken as thin streaks penetrating the reservoir vertically with high permeability along X, Y and Z- directions. Matrix block permeability values are drastically lower than those characterizing fracture block permeability. Eclipse 100 black-oil simulator was used throughout the study to analyze the effect of fracture frequency and to obtain the optimum completion by investigating different isolation length for fractured horizontal producers, injectors and both producers and injectors. The sensitivity analysis involves building numerous models for 2mD, 10mD and 50mD. That is because the range of permeability in the field is not very well defined. But, it is thought to be in the above selected range. The objective being to determine the optimum completion that gives the best performance associated with the lowest cost after several selective completion options are tested for different fracture frequency and for different type of wells (producers, injectors or both).

HORIZONTAL WELL LENGTH OPTIMIZATION FOR A SOUTH OMAN FIELD

ABSTRACT Oman produces around 850,000 bbl/day of crude oil from around 85 fields of which 35% comes from horizontal wells. Horizontal drilling was started in 1986 with three short radius wells drilled in chalky oil reservoirs due to gas or water coning and low production rates. Results were not sufficiently encouraging to lead to future activity in the short term. Horizontal drilling technology evolved rapidly and in 1990, Oman embarked on a more ambitious program. The results, this time, were so impressive that the trial was extended and has led to almost continuous horizontal drilling. Some 100 horizontal wells per year are expected to be drilled for the foreseeable future. These wells exploit both clastic and carbonate reservoirs from Precambrian to Cretaceous age, thin and thick oil columns, light and heavy oil, and a wide range of reservoir quality. Multilateral drilling in Oman has also shown significant benefits in increased production rates, reduced costs and increased oil recovery from existing producing fields and previously uncommercial oil accumulations. Well costs have been reduced by optimising the well design and drilling operations. The most significant cost saving has been due to a reduction in hole size and casing scheme. The main focus of this project is optimizing oil recovery in a South Oman oil field using multilateral well technology. It mainly outlines both the effects of horizontal length and number of horizontal legs completed for different reservoir properties using Eclipse 100 black-oil simulator as a numerical tool. Eclipse black-oil simulator is utilized in order to formulate the reservoir models of the concerned field and two other synthetic reservoirs having different rock properties. In order to generalize the findings, correlations are developed for different rock properties. That way predictions for optimum well conditions for any particular field in Oman could be established.

MATRIX AND CEMENT EFFECTS ON RESIDUAL OIL SATURATION IN SANDSTONE FORMATIONS: A NEURAL NETWORK APPROACH

Abstract Residual oil in sandstone is affected by mineral composition, clay matrix and cementing material. Matrix minerals affect the affinity of a fluid to spread on a rock surface significantly and in turn controls the fluid distribution within the pore spaces. At the interface between the rock surface and the contacting fluid, electrical charges are in the origin of the extent of phase wetness. Available framework grains, a dominant component of rock matrix, affect porosity and, hence, amounts of rock preferential wetness. Cement, clay matrix and quartz overgrowth, which make up for the rest of the grain population in a rock, influence wetness and, therefore, amounts of residual oil. In this paper, spectro-electromicroscopy (SEM) point-count technique in conjunction with neural network analysis were used to determine the effect of certains rock parameters on the amounts of residual oil following waterflooding operations. Using artifcial neural network, the intent was then to predict the extent of resid...