The study produced equations that identify six factors influencing the formation of the final gas output. The estimation of the final gas recovery coefficients for the fields of Eastern Turkmenistan was determined by calculation with the construction of “average productivity” curves. It should be noted that the type of deposit has to be taken into account to estimate the projected values of the gas recovery factors. In other words, there are large uncertainties in the estimation of the final gas recovery coefficients in the development of advanced fields.
There are many geological and technological factors influencing the final gas transfer coefficient. We can distinguish 12 factors that form the final gas output (Qdep., Play.): the ratio of the drilling area of the deposit to the total initial area of the gas content, the ratio of the minimum distance from the well to the gas content contour to the total area of the gas content, Development time, total gas extraction during the continuous production period, rate of reduction of the annual gas extraction, minimum distance from the production wells to the gas bearing circuit, formation parameters, development modes of two types and collectors of two types. Based on paired correlation and associative analyses, five of the most important factors influencing the formation of gas output were identified from the above factors:
P
0
;
The equations were obtained, in which there are six factors that influence the formation of the final gas output. Average productivity curves are also used to estimate the projected gas output. To construct this curve, the actual values of annual gas withdrawals are deferred on the coordinate axis, and on the abscisse axis the total gas selection is taken, only the values are taken as a percentage of the initial gas reserves.
Thus, comparisons of the projected gas recovery values with the actual data showed that of the existing more than twenty equations, only three with an error of 5–11 % can be used in the project development of Eastern Turkmenistan fields.
Table 1
Calculation of the final gas recovery coefficient for fields in the final stages of development.
|
Skyline |
Final gas efficiency
|
Initial formation pressure, P 0 kg/sm 2 |
Rate of decline of annual selection during falling production,
|
Maximum selection rate,
|
Deposit type |
|
1 |
2 |
3 |
4 |
5 |
6 |
|
I |
0,86 |
140,6 |
0,39 |
17,2 |
full-layer |
|
Pa |
0,72 |
162,0 |
0,47 |
8,6 |
full-layer |
|
Pb |
0,79 |
163,0 |
0,28 |
8,2 |
full-layer |
|
W |
0,52 |
170,1 |
0,34 |
8,0 |
water-flooding |
|
IUa+IUb+IUv |
0,68 |
185,2 |
0,46 |
7,6 |
full-layer |
|
Y |
0,72 |
201,8 |
0,39 |
9,7 |
full-layer |
|
UI |
0,70 |
205,9 |
0,63 |
15,0 |
water-flooding |
|
IX+X |
0,64 |
223,5 |
0,82 |
13,1 |
water-flooding |
On the basis of the treatment of the above dependencies by the method of least squares, the following equations for water and full-plastic deposits are obtained:
For water-flooding deposits:
For full-layer deposits:
Recovery factors
Oil companies will want to maximize the value of a field by getting as much of the hydrocarbons out of it as possible. However, it is not feasible to recover all of the hydrocarbons from a reservoir. Only a certain percentage of the total hydrocarbons will be recovered from a field, and this is known as the recovery factor.
Recovery factors are higher in gas fields than they are in oil fields. Typical recovery factors for gas are about 50–80 % (Jahn et al., 1998). There is more scope to improve oil recovery. Global recovery factors for oil are thought to be in the range of 30–35 % (e.g. Conn, 006). If, for example, you can recover 35 % of the oil from an oil field, why can you not produce the other 65 %? As mentioned earlier, the answer to this is not simple. The magnitude of the recovery factor for an oil field depends on a complex interplay of geological, physical, and economic elements.
A starting point is to look at the various categories of oil volumes within a typical oil field (i.e., a waterflooded oil field) and represent them on a maturity pie chart (Figure 31). These categories are residual oil, cumulative production, remaining recoverable reserves, and unrecovered mobile oil (UMO).
Residual oil saturation is the component of the oil that remains trapped within the pores after an oil-bearing sandstone has been swept by water. Somewhere between about 15 and 35 % of the total oil in sandstones can end up as residual oil.
The second category comprises ultimate recoverable oil; this is the reservoir engineer’s best estimate of what the field will produce by the predicted end of field life. This figure can be split into the volume of oil that has been produced so far (the cumulative production of hydrocarbons to date), and the estimate of what is left to produce (the reserves).
The last category is unrecovered mobile oil (UMO), oil that is movable by primary recovery or water injection, but which will be left behind at the end of field life under current reckoning (Tyler and Finley, 1991). If an oil company wants to improve the recovery factor in a field, then this category is where the oil will normally come from.
The unrecovered mobile oil can be subdivided into three subcategories (Figure 32). Target oil is oil that has a large enough volume to justify the cost of a well to recover it. The phrase locate the remaining oil has been used for the workflow involved in finding these volumes (Wetzelaer et al., 1996). This is discussed in more detail in Section 5 of this publication. Marginal oil is the category of trapped oil found in volumes just below the economic threshold to justify an infill well. These volumes will become target oil if the oil price increases or if less expensive ways can be found to access them. The third subcategory is uneconomic oil, small volumes of bypassed oil or low oil saturations that cannot be produced economically (Weber, 1999).
References:
- Fish M. L., Leyontev I. A., Hramenkov Ye.N. “Osenka koeffisientov gazootdachi v period padayushey dobychi”. Razrabotka gazovyh i gazkondensatnyh mestorojdeniy. M., VNIIEGasprom, 1974, pg 37.
- Zakirov S. N., Lapuk B. B. Proyektirovaniye i razrabotka gazovyh mestorojdeniy. M., Nedra, 1974.
- Rassohin G. V. Zavershayushaya stadiva razrabotki i ekspluatasiya gazovyh i gazkondensatnyh mestorojdeniy. M., Nedra, 1977. Scientific supervisor doctor of technical sciences, associate professor M. Gafurova.
