3.1 Reservoir Geology

Keywords: reservoir structures, faults, folds, depositional environments, diagenesis, geological controls, porosity, permeability

Introduction and Commercial Application: The objective of reservoir geology is the description and quantification of geologically controlled reservoir parameters and the prediction of their lateral variation. Three parameters broadly define the reservoir geology of a field:

• depositional environment

• structure

• diagenesis

To a large extent the reservoir geology controls the producibility of a formation, i.e. to what degree transmissibility to fluid flow and pressure communication exists. Knowledge of the reservoir geological processes has to be based on extrapolation of the very limited data available to the geologist, yet the geological model is the base on which the field development plan will be built. In the following section we will examine the relevance of depositional environments, structures and diagenesis for field development purposes.

1.0 THE FIELD LIFE CYCLE

Keywords: exploration, appraisal, feasibility, development planning, production profile,

production, abandonment, project economics, cash flow

Introduction and Commercial Application: This section provides an overview of the

activities carried out at the various stages of field development. Each activity is driven

by a business need related to that particular phase. The later sections of this book will

focus in some more detail on individual elements of the field life cycle.

Figure 1.1 The Field Life Cycle and a Simplified Business Model

1.1 Exploration Phase

For more than a century petroleum geologists have been looking for oil. During this

period major discoveries have been made in many parts of the world. However, it is

becoming increasingly likely that most of the 'giant' fields have already been discovered

and that future finds are likely to be smaller, more complex, fields. This is particularly

true for mature areas like the North Sea.

Fortunately, the development of new exploration techniques has improved geologists'

understanding and increased the efficiency of exploration. So although targets are getting

smaller, exploration and appraisal wells can now be sited more accurately and with

greater chance of success.

Despite such improvements, exploration remains a high risk activity. Many international

oil and gas companies have large portfolios of exploration interests, each with their own

geological and fiscal characteristics and with differing probabilities of finding oil or gas.

Managing such exploration assets and associated operations in many countries

represents a major task.

Even if geological conditions for the presence of hydrocarbons are promising, host

country political and fiscal conditions must also be favourable for the commercial success

of exploration ventures. Distance to potential markets, existence of an infrastructure,

and availability of a skilled workforce are further parameters which need to be evaluated

before a long term commitment can be made.

Traditionally, investments in exploration are made many years before there is any

opportunity of producing the oil (Fig. 1.2). In such situations companies must have at

least one scenario in which the potential rewards from eventual production justify

investment in exploration.

It is common for a company to work for several years on a prospective area before an

exploration well is spudded. During this period the geological history of the area will be

studied and the likelihood of hydrocarbons being present quantified. Prior to spudding

the first well a work programme will have been carried out. Field work, magnetic surveys,

gravity surveys and seismic surveys are the traditional tools employed. Section 2.0

"Exploration" will familiarise you in some more detail with the exploration tools and

techniques most frequently employed.

Figure 1.2 Phasing and expenditure of a typical exploration programme

Тимур Т.

3.2.6 Fluid sampling and PVT analysis

The collection of representative reservoir fluid samples is important in order to establish

the PVT properties - phase envelope, bubble point, R s, B o, and the physical properties

- composition, density, viscosity. These values are used to determine the initial volumes

of fluid in place in stock tank volumes, the flow properties of the fluid both in the reservoir

and through the surface facilities, and to identify any components which may require

special treatment, such as sulphur compounds.

Reservoir fluid sampling is usually done early in the field life in order to use the results

in the evaluation of the field and in the process facilities design. Once the field has been

produced and the reservoir pressure changes, the fluid properties will change as

described in the previous section. Early sampling is therefore an opportunity to collect

unaltered fluid samples.

Fluid samples may be collected downhole at near-reservoir conditions, or at surface.

Subsurface samples are more expensive to collect, since they require downhole sampling

tools, but are more likely to capture a representative sample, since they are targeted at

collecting a single phase fluid. A surface sample is inevitably a two phase sample which

requires recombining to recreate the reservoir fluid. Both sampling techniques face the

same problem of trying to capture a representative sample (i.e. the correct proportion of

gas to oil) when the pressure falls below the bubble point.

Subsurface samples

Subsurface samples can be taken with a subsurface sampling chamber, called a sampling

bomb, or with a repeat formation testing (RFT) tool or modular dynamic testing tool

(MDT), all of which are devices run on wireline to the reservoir depth. The sampling

bomb requires the well to be flowing, and the flowing bottom hole pressure (Pwf) should

preferably be above the bubble point pressure of the fluid to avoid phase segregation.

If this condition can be achieved, a sample of oil containing the correct amount of gas

(Rsi scf/stb) will be collected. If the reservoir pressure is close to the bubble point, this

means sampling at low rates to maximise the sampling pressure. The valves on the

sampling bomb are open to allow the fluid to flow through the tool and are then

hydraulically or electrically closed to trap a volume (typically 600 cm 3) of fluid. This

small sample volume is one of the drawbacks of subsurface sampling

Sampling saturated reservoirs with this technique requires special care to attempt to

obtain a representative sample, and in any case when the flowing bottom hole pressure

is lower than the bubble point, the validity of the sample remains doubtful. Multiple

subsurface samples are usually taken by running sample bombs in tandem or performing

repeat runs. The samples are checked for consistency by measuring their bubble point

pressure at surface temperature. Samples whose bubble point lie within 2% of each

other may be sent to the laboratory for PVT analysis.

Surface samples

Surface sampling involves taking samples of the two phases (gas and liquid) flowing

through the surface separators, and recombining the two fluids in an appropriate ratio

such that the recombined sample is representative of the reservoir fluid.

The oil and gas samples are taken from the appropriate flowlines of the same separator,

whose pressure, temperature and flowrate must be carefully recorded to allow the

recombination ratios to be calculated. In addition the pressure and temperature of the

stock tank must be recorded to be able to later calculate the shrinkage of oil from the

point at which it is sampled and the stock tank. The oil and gas samples are sent

separately to the laboratory where they are recombined before PVT analysis is performed.

A quality check on the sampling technique is that the bubble point of the liquid sample

at the temperature of the separator from which the samples were taken should be equal

to the separator pressure.

1.2 Appraisal Phase

Once an exploration well has encountered hydrocarbons, considerable effort will still be

required to accurately assess the potential of the find. The amount of data acquired so

far does not yet provide a precise picture of the size, shape and producibility of the

accumulation.

Two possible options have to be considered at this point:

9 to proceed with development and thereby generate income within a relatively short

period of time. The risk is that the field turns out to be larger or smaller than

envisaged, the facilities will be over or undersized and the profitability of the project

may suffer.

9 to carry out an appraisal programme with the objective of optimising the technical

development. This will delay "first oil" to be produced from the field by several

years and may add to the initial investment required. However, the overall

profitability of the project may be improved.

The purpose of development appraisal is therefore to reduce the uncertainties, in

particular those related to the producible volumes contained within the structure.

Consequently, the purpose of appraisal in the context of field development is not to find

additional volumes of oil or gas! A more detailed description of field appraisal is provided

in Section 6.0.

Having defined and gathered data adequate for an initial reserves estimation, the next

step is to look at the various options to develop the field. The objective of the feasibility

study is to document various technical options, of which at least one should be

economically viable. The study will contain the subsurface development options, the

process design, equipment sizes, the proposed locations (e.g. offshore platforms), and

the crude evacuation and export system. The cases considered will be accompanied

by a cost estimate, and planning schedule. Such a document gives a complete overview

of all the requirements, opportunities, risks and constraints.

1.3 Development Planning

Based on the results of the feasibility study, and assuming that at least one option is

economically viable, a field development plan can now be formulated and subsequently

executed. The plan is a key document used for achieving proper communication,

discussion and agreement on the activities required for the development of a new field,

or extension to an existing development.

The field development plan's prime purpose is to serve as a conceptual project

specification for subsurface and surface facilities, and the operational and maintenance

philosophy required to support a proposal for the required investments. It should give

management and shareholders confidence that all aspects of the project have been identified, considered and discussed between the relevant parties. In particular, it should include:

9 Objectives of the development

9 Petroleum engineering data

9 Operating and maintenance principles

9 Description of engineering facilities

9 Cost and manpower estimates

9 Project planning

9 Budget proposal

Once the field development plan (FDP) is approved, there follows a sequence of activities

prior to the first production from the field:

9 Field Development Plan (FDP)

9 Detailed design of the facilities

~ Procurement of the materials of construction

Fabrication of the facilities

Installation of the facilities

Commissioning of all plant and equipment

Айгуль

Reservoir rock

Reservoir rocks are either of clastic or carbonate composition. The former are composed

of silicates, usually sandstone, the latter of biogenetically derived detritus, such as coral

or shell fragments. There are some important differences between the two rock types

which affect the quality of the reservoir and its interaction with fluids which flow through

them.

The main component of sandstone reservoirs ("siliciclastic reservoirs") is quartz (SiO2).

Chemically it is a fairly stable mineral which is not easily altered by changes in pressure,

temperature or acidity of pore fluids. Sandstone reservoirs form after the sand grains

have been transported over large distances and have deposited in particular

environments of deposition.

Carbonate reservoir rock is usually found at the place of formation ("in situ"). Carbonate

rocks are susceptible to alteration by the processes of diagenesis.

The pores between the rock components, e.g. the sand grains in a sandstone reservoir,

will initially be filled with the pore water. The migrating hydrocarbons will displace the

water and thus gradually fill the reservoir. For a reservoir to be effective, the pores need

to be in communication to allow migration, and also need to allow flow towards the

borehole once a well is drilled into the structure. The pore space is referred to as porosity

in oil field terms. Permeability measures the ability of a rock to allow fluid flow through

its pore system. A reservoir rock which has some porosity but too low a permeability to

allow fluid flow is termed "tight".

In Section 5.1 we will examine the properties and lateral distribution of reservoir rocks

in detail.

Traps

Hydrocarbons are of a lower density than formation water. Thus, if no mechanism is in

place to stop their upward migration they will eventually seep to the surface. On seabed

surveys in some offshore areas we can detect crater like features ("pock marks") which

also bear witness to the escape of oil and gas to the surface. It is assumed that throughout

the geologic past vast quantities of hydrocarbons have been lost in this manner from

sedimentary basins.

There are three basic forms of trap as shown in Figure 2.5. These are:

Anticlinal traps which are the result of ductile crustal deformations

Fault traps which are the result of brittle crustal deformations

Stratigraphic traps where impermeable strata seals the reservoir

In many oil and gas fields throughout the world hydrocarbons are found in fault bound

anticlinal structures. This type of trapping mechanism is called a combination trap.

Figure 2.5 Main Trapping Mechanisms

Even if all of the elements described so far have been present within a sedimentary

basin an accumulation will not necessarily be encountered. One of the crucial questions

in prospect evaluation is about the timing of events. The deformation of strata into a

suitable trap has to precede the maturation and migration of petroleum. The reservoir

seal must have been intact throughout geologic time. If a "leak" occurred sometime in

the past, the exploration well will only encounter small amounts of residual hydrocarbons.

Conversely, a seal such as a fault may have developed early on in the field's history

and prevented the migration of hydrocarbons into the structure.

In some cases bacteria may have "biodegraded' the oil, i.e. destroyed the light fraction.

Many shallow accumulations have been altered by this process. An example would be

the large heavy oil accumulations in Venezuela.

Given the costs of exploration ventures it is clear that much effort will be expended to

avoid failure. A variety of disciplines are drawn in such as geology, geophysics mathematics, and geochemistry to analyse a prospective area. However, on average,

even in very mature areas where exploration has been ongoing for years, only every

third exploration well will encounter substantial amounts of hydrocarbons. In real 'wildcat'

areas, basins which have not been drilled previously, only every tenth well is, on average,

successful.

Тимур У.

Field studies

There is only one method available that allows the study of the vertical and lateral

relationship of the different rock types of a reservoir on a scale of 1:1. This is the study

of outcrops. These are areas like quarries, roadcuts, cliffs, mines, etc., which consist of

a sequence known to be a reservoir in the vicinity or the lateral equivalent thereof.

Detailed investigation of a suitable outcrop can often be used as a predictive tool to

model:

9 presence, maturity and distribution of source rock

9 porosity and permeability of a reservoir

9 detailed reservoir framework, including flow units, barriers and baffles to fluid flow

9 frequency, orientation and geological history of fractures and sub-seismic faults

9 lateral continuity of sands and shales

9 quantitative description of all of the above for numerical reservoir simulations

Over the last decade some of the major oil companies have been using vast amounts

of outcrop derived measurements to design and calibrate powerful computer models.

These models are employed as tools to quantitatively describe reservoir distribution

and flow behaviour within individual units. Hence this technique is not only important for

the exploration phase but more so for the early assessment of production profiles.

Mudlogging

The technique of mudlogging is covered in this section because it is one of the first

direct evaluation methods available during the drilling of an exploration well. As such,

the mudlog remains an important and often under-used source of original information.

This first information about the reservoir is recorded, as a function of depth, in the form

of several columns. Although rather qualitative in many respects, mudlogging is an

important data gathering technique. It is of importance as a basis for operational

decisions, e.g. at what depth to set casing, or where to core a well. Mudlogging is also

cheap, as data is gathered while the normal drilling operations go on.

The rate at which the drill bit penetrates the formation gives qualitative information

about the lithology being drilled. For example, in a hard shale the rate of penetration

(ROP) will be slower than in a porous sandstone.

Figure 2.14

The formation cuttings that are chipped off by the bit travel upward with the mud and

are caught and analysed at the surface. This provides information about the lithology

and qualitative indications of the porosity.

If there are hydrocarbons present in the formation that is being drilled, they will show in

the cuttings as oil stains, and in the mud as traces of oil or gas. The gas in the mud is

continuously monitored by means of a gas detector. This is often a relatively simple

device detecting the total combustible gas content. The detector can be supplemented

by a gas chromatograph, which analyses the composition of the gas.

Figure 2.14 shows an example of a basic mudlog, including information about the drilling

rate, cuttings and hydrocarbon "shows". The sands clearly show up on both the drilling

rate and the cuttings description. Oil stains were observed in the cuttings, and the gas

detector gives high readings and indicates the presence of heavy components in the

gas. This example illustrates that the value of a mudlog lies in the combination of the

information received from the various sources.

A mudlog provides only qualitative information, hence it is unsuitable for an accurate

formation evaluation. Mudlogging is therefore nowadays partly replaced by logging while

drilling techniques (LWD) which will be covered in Section 5.3.

In summary, exploration activities require the integration of different techniques and

disciplines. Clear definition of survey objectives is needed. When planning and executing

an exploration campaign the duration of data acquisition and interpretation has to be

taken into account.

Руслан , Дарья 8.4

Ильдар8.5

Азамат8.6

Ильнур8.7

Давид8.8

Рустам 15.4