Gathering

Table 7.8, which gathers the French government specifications and test methods concerning hydrocarbon solvents, is divided into three parts ... 

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. 

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 section is divided into four parts, which discuss the common reservoir types from a geological viewpoint, the fluids which are contained within the reservoir, the principal methods of data gathering and the ways in which this data is interpreted. Each section is introduced by pointing out its commercial relevance. 

To derive a reservoir geological model various methods and techniques are employed mainly the analysis of core material, wireline logs, high resolution seismic and outcrop studies. These data gathering techniques are further discussed in Sections 5.3 and 2.2. 

Data gathering in the water column should not be overlooked at the appraisal stage of the field life. Assessing the size and flow properties of the aquifer are essential in predicting the pressure support which may be provided. Sampling of the formation water is necessary to assess the salinity of the water for use in the determination of hydrocarbon saturations. 

The basic data gathering methods are direct methods which allow visual inspection or at least direct measurement of properties, and indirect methods whereby we infer reservoir parameters from a number of measurements taken in a borehole. The main techniques available within these categories are summarised in the following table ... 

This section will look at formation and fluid data gathering before significant amounts of fluid have been produced hence describing how the static reservoir is sampled. Data gathered prior to production provides vital information, used to predict reservoir behaviour under dynamic conditions. Without this baseline data no meaningful reservoir simulation can be carried out. The other major benefit of data gathered at initial reservoir conditions is that pressure and fluid distribution are in equilibrium this is usuaily not the case once production commences. Data gathered at initial conditions is therefore not complicated... 

Mudlogging is another important direct data gathering technique, which was discussed in some detail in Section 2.2, Exploration Methods and Techniques. 

A common objective of a data gathering programme is the acquisition of fluid samples. The detailed composition of oil, gas and water is to some degree reguired by almost every discipline involved in field development and production. 

Having gathered and evaluated relevant reservoir data it is desirable to present this data in a way that allows easy visualisation of the subsurface situation. With a workstation it is easy to create a three dimensional picture of the reservoir, displaying the distribution of a variety of parameters, e.g. reservoir thickness or saturations. All realisations need to be in line with the geological model. 

It is worth noting that if field development using horizontal wells is under consideration, then horizontal appraisal wells will help to gather representative data and determine the benefits of this technique, which is further discussed in Section 9.3. 

The first function of a wellsite is to accommodate drilling operations. However, a wellsite must be designed to allow access for future operations and maintenance activity, and in many cases provide containment in the event of accidental emission. Production from a single wellhead or wellhead cluster is routed by pipeline to a gathering station, often without any treatment. In such a case the pipeline effectively becomes an extension of the production tubing. If a well is producing naturally or with assistance from a down... 

The term gathering station may describe anything from a very simple gathering and pumping station to a complex processing centre in which produced fluids are treated and separated into gas, NGLs and stabilised crude. 

If several widely spaced fields are feeding a single gathering and treatment centre it is common to perform primary separation of gas and oil (and possibly water) in the field. A field station may include a simple slug catcher, temporary storage tanks and pumps for getting the separated fluids to the main gathering and treatment centre. 

A complex gathering station may include facilities to separate produced fluids, stabilise crude for storage, dehydrate and treat sales gas, and recover and fractionate NGLs. Such a plant would also handle the treatment of waste products for disposal. 

Sales gas would be piped directly into the national gas distribution network (assuming one exists) and NGL products such as propane and butane can be stored locally in pressurised tanks. NGL products are often distributed by road or rail directly from the gathering station, although if ethane is recovered it is normally delivered by pipeline. 

The function of offshore production facilities are very much the same as those described for land operations. An offshore production platform is rather like a gathering station hydrocarbons have to be collected, processed and evacuated for further treatment or storage. However, the design and layout of the offshore facilities are very different from those on land for the following reasons ... 

As discussed in Section 13.2, the technical, fiscal and economic data gathered to construct a project cashflow carry uncertainty. An economic base case is constructed using, for example, the most likely values of production profile and the 50/50 cost estimates, along with the best estimate of future oil prices and the anticipated production agreement and fiscal system. 

A programme of monitoring the reservoir is carried out, in which me asurements are made and data are gathered. Figure 14.2 indicates some of the tools used to gather data, the information which they yield, and the way In which the information is fed back to update the models and then used to refine the ongoing reservoir development strategy. 

Some specific examples of the use of data gathered while monitoring the reservoir will now be discussed. 

The data gathered from the logs and cores of the development wells are used to refine the correlation, and better understand areal and vertical changes in the reservoir quality. Core material may also be used to support log data in determining the residual hydrocarbon saturation left behind in a swept zone (e.g. the residual oil saturation to water flooding). 

During the design phase, facilities (the hardware items of equipment) are designed for operating conditions which are anticipated based upon the information gathered during field appraisal, and upon the outcome of studies such as the reservoir simulation. The design parameters will typically be based upon assessments of... 

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