Shivaji N. Dasgupta

Fundamentals of Petroleum Geology

Abstract Origin of petroleum begins with the formation of organic matter, burial of organic matter in a basin maturation of the organic content with pressure temperature at burial depths. Petroleum system includes source rocks, reservoir rocks, reservoir traps, migration paths, seals etc. Hydrocarbons mature in source rocks migrate into reservoirs. Reservoir rocks are containers of hydrocarbons with sufficient interconnected pore spaces, these are sedimentary rocks clastic (sandstone shale), carbonate rocks. Migration pathways for matured hydrocarbons- migration is in separate phases from higher potential to a lower potential, from deepest to the shallowest. The hydrocarbons migrate into different types of petroleum traps such as anticlinal, fault, salt related stratigraphic traps. Reservoir seals are rocks with low permeability drapes hydrocarbons traps to ensure that entrapped fluids do not escape. Integration of disciplines geology, geophysics, petrophysics, is the key to predicting reservoir geometry, volume, in assessment of reserves.

From Seismic Character and Seismic Attributes to Reservoir Properties: Case History in Arab-D Reservoir of Saudi Arabia

SUMMARY Seismic Attributes and subtle changes in the character of the seismic response from a target horizon are used to predict different reservoir properties. Among the key reservoir properties that are predicted are the interwell porosity, lithology, heterogeneity and fluid saturation. A catalogue of seismic characters for different representative wells in Arab D reservoir of Ghawar oil field is developed. A neural network is trained to classify those seismic characters to different groups based on to the corresponding reservoir properties and geologic facies. The entire 3-D data cube in the study area is then divided to different classes based on the proximity of their respective seismic character to those of the catalogue. To further improve the resolving power of seismic characters, other attributes that are considered to be sensitive to given reservoir properties are combined with the seismic character for classification purposes. Aside from some of the conventional attributes, fluid sensitive attributes that capture the hydrocarbon related high frequency loss are one set of attributes used. Others are the time frequency analysis (TFA) attribute and the porosity sensitive acoustic impedance attributes. GEOLOGIC SETTING Ghawar, the largest oil field in the world is located onshore, east of Saudi Arabia. It is approximately 250 Kilometers north-south and 30 Kilometers east-west. The field was discovered in 1936. The upper Jurassic Arab-D carbonate reservoir in Ghawar field is cyclical and heterogeneous. It is a limestone reservoir with a wide range to very high porosity/permeability (with numerous super K production profiles in some parts of the field). Horizontal wells are drilled in the Arab-D reservoir to enhance well productivity and fluid injectivity. These wells are designed to produce by-passed or trapped oil from zones underlain by water, to produce selected tight layers in the complex a vertical aggrading and prograding carbonate reservoir. Understanding the fracture and fault distribution, reservoir heterogeneity and fluid saturation is critical for planning and completion of the horizontal wells. While it is desirable to have the wells intersect the fractures in the low permeable reservoir layers, this should be avoided where fractures form conduits for premature break-through water. METHOD OF ATTACK

Chapter 4 - Formation Evaluation

Petrophysical analysis of well logs and cores provide information about formation rocks and fluids in the borehole. Various types of well logs measure different properties in the well. Analysis of the data determines the volume of hydrocarbons present in a reservoir, and its potential to flow through the reservoir rock into the wellbore. This helps us to understand and optimize the producibility of a reservoir. When oil and gas wells are drilled, physical property measurements are taken using specialized geophysical instrument packages: either on wireline cables after the drill pipe has been removed (wireline logs), or from the borehole while drilling with instruments attached to drill collars (LWD).

Chapter 7 - Reservoir Monitoring

Monitoring or surveillance of reservoir fluids during the producing life of a field and mapping of oil–water and gas–oil interface are necessary for understanding the fluid dynamics. This information would allow improving developmental plans, locate production and injection wells, and optimize reservoir management. Engineers would like to have information between the well control for field-wide surveillance of fluid front. This would permit forecasting arrival of water in a producing well or understanding the effectiveness of sweep from the injected water or gas. Geophysical monitoring could have major impact on ultimate recovery and drilling efficiency.

Chapter 6 - Reservoir Characterization

Accurate reservoir characterization is a key step in developing, monitoring, and managing a reservoir and optimizing production. To achieve accuracy and to ensure that all the information available at any given time is incorporated in the reservoirmodel, reservoir characterizationmust be dynamic. To achieve this goal, however, one starts with a simple model of the reservoir at a given time point (a static model). As new petrophysical, seismic, and production data become available, the reservoir model is updated to account for the changes in the reservoir. The updated model would be a better representative of the current status of the reservoir. Both static reservoir properties, such as porosity, permeability, and facies type; and dynamic reservoir properties, such as pressure, fluid saturation, and temperature, needs to be updated as more field data become available. Characterizing a reservoir by updating of both static and dynamic reservoir properties during the life of the field is referred to as dynamic reservoir characterization. Dynamic reservoir characterization is discussed in Chapter 7, dealing with time lapse or 4D geophysical data and reservoir monitoring. This chapter, however, focuses on static reservoir characterization.

Chapter 8 - Geophysics in Drilling

Abstract The process of drilling an oil or gas well requires knowledge of all geologic features expected to be encountered along the way—from the surface of the ground to the target reservoir. Thus, in addition to steering the well so as to intersect hydrocarbon-bearing reservoirs, the reservoir engineer must assure to a reasonable degree of confidence that the well drills successfully and safely to the target. Geophysical measurements help ensure a successful drilling program. 3D seismic provides a picture of the subsurface from the surface to the target.

Chapter 5 - Geostatistics and Other Unconventional Statistical Methods

Abstract In most oil exploration and production problems, we deal with limited and incomplete data. We are constantly trying to extrapolate information from sparse measurements, for example, sparse well data and limited core measurements on the one hand and large volumes of seismic data with limited spatial resolution on the other hand. We resort to statistical methods to accomplish the data extrapolation and the integration of diverse data sets in constructing a coherent and meaningful model of the subsurface. Traditional statistical methods both for spatial and temporal extrapolation have been used in E&P for several decades. One of the main uses of statistics has been for reservoir characterization through integrating information and data from various sources with varying degrees of uncertainty such as log, well tests, and seismic data. Other applications include establishing relationships between measurements and reservoir properties, and reserve estimation and oil field economics along with the associated risk factors.

Fundamentals of Petroleum Geophysics

Abstract Geophysical techniques apply the principles of physics for study of physical responses of rocks under passive or active perturbation. Geophysical data acquisition, processing and interpretation are driven by established scientific principles. Data from geophysical tools provide coverage with spatially continuous high density measurements. Well data like cores and well logs provide vertically high resolution measurements at the well location, however, the distribution of wells is sparse and discontinuous. The detailed spatial coverage from geophysical data are calibrated with analysis of well logs, pressure tests, cores, geologic depositional knowledge and other information from appraisal wells. The methods use high precision sensors that measure the properties on the earths surface, in oceans, in wells and from the air, also from satellites. They measure changes of physical properties and calibrate the measured geophysical attributes with rock properties. The data play important role in developing a gross reservoir model. The reservoir architecture or structure and the reservoir rock and fluid properties are derived from the analysis and data integration. Other reservoir properties that can affect geophysical measurements are density, oil viscosity, stresses, and fractures. The interpretation has inherent ambiguity or multiple interpretations. Geophysics contributes to reservoir characterization, reservoir monitoring and its management by adding maximum value in improving production plan and by minimizing risk e.g., risk of dry hole, risk of blow out, risk of in-efficient recovery process, among others. Multiple geologic parameters are assessed with the same geophysical data.

Geophysics for Unconventional Resources

Abstract Geophysical methods are being applied that are specifically relevant to the exploration of and production from unconventional reservoirs. While many of the techniques have common applications for both conventional and unconventional reservoirs, there are also some significant differences in focus. Much of the unconventional reservoirs are from shale formations. Characterizing of fracture system in such reservoirs is of prime importance not only to identify the “sweet spots” for well placement but also for optimum drilling and production from such fractured reservoirs. Combining conventional and microearthquake seismic data has proven to improve the characterization process. Another factor for drilling through shale reservoirs is the need for stimulation through hydraulic fracturing. Use of microseismic data for monitoring the frac process has gained prominence in recent years. Different types of design to acquire such geophysical data and to utilize them for multistage fracking process as well as their integration with the conventional seismic data have been developed.

Geophysical Reservoir Monitoring Forum in the Arabian Peninsula

Saudi Aramco and Petroleum Development Oman (PDO) cosponsored the Geophysical Reservoir Monitoring Forum, Arabian Peninsula, held in Bahrain on 15–17 March 2005. The objective was for the regions national oil companies, international oil companies, service companies, and academic institutions to share their experiences with geophysical reservoir monitoring in the region, and also knowledge gained in other parts of the world that is pertinent to the Arabian Peninsula. Some issues of particular interest in the region are carbonate reservoirs, noisy land seismic data, large well spacing, and low reservoir depletion rates.