Declination

English Culture and the Decline of the Industrial Spirit Martin J. Wiener... 

Initially new wells may still be brought on stream but the older wells start to decline. A constant production rate is maintained. This period is typically 2 to 5 years for an oil field, but longer for a gas field. 

During this final (and usually longest) period all producers will exhibit declining production. 

The value of the compresjiibility of oil is a function of the amount of dissolved gas, but is in the order of 10 x 10" psi" By comparison, typical water and gas compressibilities are 4x10" psi" and 500 x 10" psi" respectively. Above the bubble point in an oil reservoir the compressibility of the oil is a major determinant of how the pressure declines for a given change in volume (brought about by a withdrawal of reservoir fluid during production). 

Keywords compressibility, primary-, secondary- and enhanced oil-recovery, drive mechanisms (solution gas-, gas cap-, water-drive), secondary gas cap, first production date, build-up period, plateau period, production decline, water cut, Darcy s law, recovery factor, sweep efficiency, by-passing of oil, residual oil, relative permeability, production forecasts, offtake rate, coning, cusping, horizontal wells, reservoir simulation, material balance, rate dependent processes, pre-drilling. 

Once the bubble point is reached, solution gas starts to become liberated from the oil, and since the liberated gas has a high compressibility, the rate of decline of pressure per unit of production slows down. 

In the solution gas drive case, once production starts the reservoir pressure drops very quickly, especially above the bubble point, since the compressibility of the system is low. Consequently, the producing wells rapidly lose the potential to flow to surface, and not only is the plateau period short, but the decline is rapid. 

This rather low recovery factor may be boosted by implementing secondary recovery techniques, particularly water Injection, or gas injection, with the aim of maintaining reservoir pressure and prolonging both plateau and decline periods. The decision to implement these techniques (only one of which would be selected) Is both technical and economic. Technical considerations would be the external supply of gas, and the... 

The aquifer response (or impact of the water injection wells) may maintain the reservoir pressure close to the initial pressure, providing a long plateau period and slow decline of oil production. The producing GOR may remain approximately at the solution GOR if the reservoir pressure is maintained above the bubble point. The outstanding feature of the production profile is the large increase in water cut over the life of the field, which is usually the main reason for abandonment. Water cut may exceed 90% in the final part of the field life. As water cut increases, so oil production typically declines a constant gross liquids (oil plus water) production may be maintained. 

A comfortable margin is maintained between the flowing tubing head pressure (downstream of compression) and the minimum pressure required for export, since the penalties for not meeting contract quantities can be severe. The decision not to install a fourth stage of compression in the above example is dictated by economics. During the final part of the pressure decline above, the field production is of course also declining. 

The production profile for oil or gas is the only source ofrevenueior most projects, and making a production forecast is of key importance for the economic analysis of a proposal (e.g. field development plan, incremental project). Typical shapes of production profile for the main drive mechanisms were discussed in Section 8.2, but this section will provide some guidelines on how to derive the rate of build-up, the magnitude and duration of the plateau, the rate of decline, and the abandonment rate. 

Once the production potential of the producing wells is insufficient to maintain the plateau rate, the decline periodbegins. For an individual well in depletion drive, this commences as soon as production starts, and a plateau for the field can only be maintained by drilling more wells. Well performance during the decline period can be estimated by decline curve analysis which assumes that the decline can be described by a mathematical formula. Examples of this would be to assume an exponential decline with 10% decline per annum, or a straight line relationship between the cumulative oil production and the logarithm of the water cut. These assumptions become more robust when based on a fit to measured production data. 

In gas field development, the recovery factor is largely determined by how low a reservoir pressure can be achieved before finally reaching the abandonment pressure. As the reservoir pressure declines, it is therefore common to install compression facilities at the surface to pump the gas from the wellhead through the surface facilities to the delivery point. This compression may be installed in stages through the field lifetime. 

Artificial lift systems are mostly required later in a field s life, when reservoir pressures decline and therefore well productivities drop. If a situation is anticipated where artificial lift will be required or will be cost effective later in a field s life, it may be advantageous to install the artificial lift equipment up front and use it to accelerate production throughout the field s life, provided the increased revenues from the accelerated production offset... 

Maintenance costs account for a large fraction of the total operating expenditure (opex) of a project. Because of the bath tub curve mentioned above, maintenance costs typically increase as the facilities age just when the production and hence revenues enter into decline. The measurement and control of opex often becomes a key issue during the producing lifetime of the field as discussed in Section 14.0. However, the problem should be anticipated when writing the FDP. 

Any opex estimate should not ignore the cost of overheads which the project attracts, especially for example the cost of support staff and office rental which can form a significant fraction of the total opex, and does not necessarily reduce as production declines. 

In Section 13.2, it was suggested that opex is estimated at the development planning stage based upon a percentage of cumulafive capex (fixed opex) plus a cosf per barrel of hydrocarbon production (variable opex). This method has been widely applied, with the percentages and cost per barrel values based on previous experience in the area. One obvious flaw in this method is that as oil production declines, so does the estimate of opex, which is nof the common experience as equipment ages it requires more maintenance and breaks down more frequently. 

Keywords production decline, economic decline, infill drilling, bypassed oil, attic/cellar oil, production potential, coiled tubing, formation damage, cross-flow, side-track, enhanced oil recovery (EOR), steam injection, in-situ combustion, water alternating gas (WAG), debottlenecking, produced water treatment, well intervention, intermittent production, satellite development, host facility, extended reach development, extended reach drilling. 

Introduction and Commercial Application The production decline period for a field is usually defined as starting once the field production rate falls from its plateau rate. Individual well rates may however drop long before field output falls. This section introduces some of the options that may be available, initially to arrest production decline, and subsequently to manage decline in the most cost effective manner. 

The field may enter into an economic decline when either income is falling (production decline) or costs are rising, and in many cases both are happening. Whilst there may be scope for further investment in a field in economic decline, it should not tie up funds that can be used more effectively in new projects. A mature development must continue to generate a positive cashflow and compete with other projects for funds. The options that are discussed in this section give some idea of the alternatives that may be available to manage the inevitable process of economic decline, and to extend reservoir and facility life. 

A considerable percentage (40% - 85%) of hydrocarbons are typically not recovered through primary drive mechanisms, or by common supplementary recovery methods such as water flood and gas injection. This is particularly true of oil fields. Part of the oil that remains after primary development is recoverable through enhanced oil recovery (EOR) methods and can potentially slow down the decline period. Unfortunately the cost per barrel of most EOR methods is considerably higher than the cost of conventional recovery techniques, so the application of EOR is generally much more sensitive to oil price. 

The physical reasons for the benefits of EOR on recovery are discussed in Section 8.7, and the following gives a qualitative description of how the techniques may be applied to manage the production decline period of a field. 

The character of a satellite development has considerable implications for a mature field in decline, but will not always have a positive economic effect on the life of the host. The remainder of this section will address the advantages of incremental development from the perspective of managing decline. 

Initially, if operating costs can be divided based on production throughput, the satellite development project may look attractive. However, the unit costs of the declining host field will eventually exceed income and the satellite development may not be able to support the cost of maintaining the old facilities. If the old facilities can be partly decommissioned, and provision made for part of the abandonment cost, then the satellite development may still look attractive. The satellite development option should always be compared to options for independent development. 

If the normal carbonate is used, the basic carbonate or white lead, Pb(OH),. 2PbCO,. is precipitated. The basic carbonate was used extensively as a base in paints but is now less common, having been largely replaced by either titanium dioxide or zinc oxide. Paints made with white lead are not only poisonous but blacken in urban atmospheres due to the formation of lead sulphide and it is hardly surprising that their use is declining. 

The importance of all this work lies in the fact that it established for the first time that chemical reactivity data for a wide series of reactions can be put onto a quantitative footing. With continuing research, however, it was found that the various chemical systems required quite specific substituent constants of their own, leading to a decline in interest in LEER. Nevertheless, substituent constant scales are still in use and methods for calculating or correlating them are still of interest [12]. 

Thus, the principal components are constructed in order of declining importance the first principal component comprise.s as much of the total variation of all variables as possible, the second principal component as much of the remaining variation, and so on. 

In the earlier versions of the streamline upwinding scheme the modified weight function was only applied to the convection tenns (i.e. first-order derivatives in the hyperbolic equations) while all other terms were weighted in the usual manner. This is called selective or inconsistent upwinding. Selective upwinding can be interpreted as the introduction of an artificial diffusion in addition to the physical diffusion to the weighted residual statement of the differential equation. This improves the stability of the scheme but the accuracy of the solution declines. 

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