saturated oil reservoir1 completo

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saturated oil reservoirs

1 introduction

the material balance equations discussed in chapter 5 apply to volumetric and water-drive reservoirs in which there

is no initial gas cap (i.e., they are initially undersaturated). however, the equation apply to reservoirs in which an

artidicial gas caps forms owing either to gravitational segregation of the iol and free gas phases below the bubble

point, or to the. inyection of gas, usually in the higher structural portions of the reservoir. when there is an initial gas

cap (i.e., the iol is initially saturated), there is negligible liquid expansion energy. however, the energy storedin the

dissolved gas is supplemented by that in the cap, and it is not surprising that recoveries from gas cap reservoirs are

generally higher than from those without caps, other things remaining equal. in gas cap drives, as production

proceeds and reservoir pressure declines, the expansion of the gas displaces oil downward toward the wells. this

phenomeon is observed in the increase of the gas-oil ratios in successively lower wells. at the same time, by virtue of

its expansion, the gas cap retards pressure decline and therefore the liberation of solution gas within the oil zone,

thus improving recovery by reducing the producing gas-oil ratios of the wells. this mechanism is most effective in

those reservoirs of marked structural relief, which introduces a vertical component of fluid flow whereby

gravitational segregation of the oil and free gas in the sand may occur. the recoveries from volumetric gas cap

reservoirs could range from the recoveries for undersaturated reservoirs up to 70 to 80% of the initial stock tank oilin place and will be higher for large gas caps, continuos uniform formations, and good gravitational segregation

characteristics.

large gas caps

the size of the gas cap is usually expressed relative to the size of the oil zone by the ratio m, as defined in chapter 2.

continuous uniform formations

continuous uniform formations reduce the channeling of the expanding gas cap ahead of the oil and the bypassing of

oil in the less permeable portions.

good gravitational segregation characterjsctics

these characteristics include primarily (a) pronounced structure, (b) low oil viscosity, (c) high permeability, and (d)

low oil velocities.

water drive and hydraulic control are terms used in designating a mechanism that involves the movement of water

into the reservoir as gas and oil are produced. water influx into a reservoir may be edgen water zone of sufficient

thickness so that the water drive is a result of expansion of the water and the compressibility of the rock in the

aquifer; however, it may result from artesian flow. the important characterjstjcs of a water-drive recovery process

are the following:

1. the volume of the reservoir is constantly reduced by the water influx. this influx is a source of energy in addition to

the energy of liquid expansion above the bubble point and the energy stored in the solution gas and in the free, or

cap, gas.

2. the bottom-hole pressure is related to the ratio of water influx to voidage. when the voidage only slightly exceeds

the influx, there is only a slight pressure decline is pronounced and approaches that for gas cap or dissolved gas-

drive reservoirs, as the case may be.

3. for edge-water drives, regional migration is pronounced in the direction of the higher structural areas.

4. as the water encroaches in both edge-water and bottom-water drives, there is an increasing volume of water

preduced, and eventually water is produced by all wells.


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* references thoughout the text are given at the end of each chapter.

5. under favorable conditions, the oil recoveries are high and range from 60 to 80% of the oil in place.

2. material balance in saturated reservoirs

the general schiltuis material balance equatin was developed in chapter 2 and is as follows:

FORMULA

equation (2.7) can be rearranged and solved for N, the initial oil in place:

FORMULA

if the expansion term due to the compressibilities of the formation and connate water can be neglected, as they

usually are in a saturated reservoir, then eq. (6.1) becomes

FORMULA

example 6.1 shows the application of eq. (6.2) to the calculation of initial oil in place for a water-drive reservoir with

an initial gas cap. the calculations are done once by converting all barrel units to cubic feet units and then converting

all cubic feet units to barrel units. it does not matter which set of units is used, only that each term in the equation is

consistent. problems sometimes arise because gas formation volume factors are either reported in cu ft/SCF or in

reservoir, gas formation volume factors are reported in bbl/SCF. use care in making sure that the units are correct.

example 6.1 to calculate the stock tank barreis of oil initially in place in a combination drive reservoir.

given:

volume of bulk oil zone = 112,000 ac-ft

volume of bulk gas zone = 19,600 ac-ft

initial reservoir pressure 2710 psia

initial FVF = 1.340 bbl/STB

initial gas volume factor = 0,006266 cu ft/STB

oil produced during the interval = 20 MM STB

reservoir pressure at the end if the interval = 2000 psia

average produced GOR = 700 SCF/STB

two-phase FVF at 2000 psia = 1,4954 bbl/STB

volume of water encroached = 11.58 MM bbl

volume of water produced = 1,05 MM STB

FVF of the water = 1,028 bbl/STB

gas volume factor at 2000 psia = 0m008479 cu ft/SCF


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solution: in the use of eq.(6.2):

B= cu ft/STB

B=

W=

W= res cu ft

assuming the same porosity and connate water for the oil and gas zones:

FORMULA m=

substituting in eq. (6.2):

FORMULA

if B is in barrels per stock tank barrel, then Bg must be in barrels per standard cubic foot and We and Wp in barrles,

and the substitution is as follows:

EJERCICIO asta aki repetir el ejercicio 6.1 cinco veces

in chapter 2, the concept of drive indexes, first introduced to the reservoir engineering literature by pirson , was

developed. to illustrate the use of these drive indexes, calculations are performed on the conroe field, texas.

figure 6.1 shows the pressure and production history of the conroe field, and fig. 6.2 gives the gas and two-phase oil

formation volume factor for the reservoir fluids. table 6.1 contains other reservoir and production data andsummarizes the calculation in columm form for three different periods.

GRAFICO

fig. 6.1 reservoir pressure and production data, conroe field (after schilthuis, trans. AIME.)

GRAFICO

fig. 6.2 pressure volume relations for conroe field oil and original complement of dissolved gas. (after schilthuis,

trans. AIME.)

TABLA 6.1

the use of such tabular forms is common in many calculations of reservoir engineering in the interest of

standardizing and summarizing calculations that may not be reviewed or repeated for intervals of months or

sometimes longer. they also enable an engineer to take over the work of a predecessor with a minimuj of briefing

and study. tabular forms also have the advantage of providing at a glance the component parts of a calculation,

many of which have singnificance themselves. the more important factors can be readily distinguished from the lessiportant ones, and trends in some of the component parts often provide imsight into the reservoir behavior. for

example, the values of line 11 in table 6.1 show the expansion of the gas cap of the conroe field as the pressure

declines. line 17 shows the values and others calculated elsewhere are plotted versus cumulative production in fig.


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for the example of the conroe field in the previous section, the water production values were not known. for this

reason, two dummy parameters are defined as F' = F - WpBw and W'e = We-WpBw. equation (6.4) then becomes

FORMULA

equation (6.5) is now in the desired form. if a plot of F'/[Eo+mB.........] as the ordinate and W'e/[ Eo+mB.......] as the

abscissa is constructed, a straight line with slope equal to 1 and intercept equal to N is obtained. table 6.2 contains

the calculated values of the ordinate, line 5, and abscissa, line 7, using the conroe field data from table 6.1. figure 6.4

is a plot of these values.

if a least squares regression analysis is done on all there data points calculated in table 6.2, the result is the solid line

shown in fig. 6.4. the line has a slope of 1.21 and an intercept, of N, of 396 MM STB. this slope is significantly larger

than 1, which is what we should have obtained from the havlena-odeh method. if we now ignore the first data point,

which represents the earliest production, and determine the slope and intercept of a line drawn through the

remainig two points (the dashed line in fig. 6.4), we get 1.00 for

table 6.2.

tabuleted values from the conroe field for use in the havlena-odeh method

line No. quantity units months after start of production

GRAFICO

fig.6.4. havlena-odeh plot for the conroe field. solid line represents line drawn through all the data points. dashed

line represents line drawn through data points from the later production periods.

a slope and 600 MM STB for N, the intercept. this value of the slope meets the requirement for the havlena-odeh

method for this case. we should now raise the question: can we justify ignoring the first point? if we realize that the

production represents less than 5% of the initial oil in place and fact that we have met the requirement for the slope

of 1 for this case, then there is justification for not including the first point in our analysis.we conclude from our

analysis that the initial oil in place is 600 MM STB for the conroe field.

you may take issue with the fact that an analysis was done on only two points. clearly, it would have been better to

use more data points, but none were available in this particular example. as more production data are colleceted,then the plot in fig. 6.4 can be updated and the calculation for N reviewed. the important point to remember is that

if the havlena-odeh method is used, the condition of tje slope and/or intercept must be met for the particular case

you are working with. this imposes another restriction on the case of the conroe field example.

saturated oil reservoir1 completo

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