Reservoir pressure measurements 1
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1) The document discusses methods for determining reservoir pressure gradients, depths of fluid interfaces like gas-oil and oil-water contacts, and pressures at various points from measured well data. 2) Equations are provided to calculate pressure gradients between two depths, depths of hydrocarbon interfaces using gradients above and below the interface, and pressures at perforations or contacts using the gradient and depth. 3) An example problem demonstrates calculating gradients, gas-oil contact depth, oil-water contact depth, and pressures at the contacts and perforations using a table of depth and pressure measurements.
Reservoir pressure measurements 1
- 2. Pressure gradient in the well Pressure gradient /m = --------------------------------------------------------------------------------- Pressure at higher depth (H 1) - ( Pressure at lower depth (H 2 ) ( H 1 ) - ( H 2 ) ------------------------------ (1) Pressure gradient /10 m =-------------------------------------- P1500 - P1490 10 -------- (2) Where P1500 and P1400 are the example pressure at 1500m and 1490 m depths respectively THE PRESSURE GRADIENT PER TEN METERS UNDER THE STATIC CONDITIONS IS APPROXIMATLY EQUAL TO THE SPECIFIC GRAVITY OF THE LIQUID. example 1
- 3. Reduction of pressure at common datum The static pressures are reduced to common depth (datum). Usually it is mid of horizon or mid of oil column. (mid of top of oil bearing sand and OWC for oil reservoirs.) The relation may be given as: P1 = P2 + (H1 - H2 )x (pressure gradient/m) Where P1 and P2 are pressures in atmospheres at depths H1 and H2 ------------------------------ (3) 2
- 4. Depth of gas-oil ,oil water interface- GOC and OWC Gradient data provide a mean for selecting the intervals in which fluid column changes from gas to oil or from oil to water. Once the intervals are selected, depth of the hydrocarbon interface (Hc) can be calculated by using the following formula: Hc = H1 + ------------------------------------ (Pb - Pt) - Gdh H Hc = Depth to interface, m H1 = Depth to top of interval, in which interval occurs, m Pb = pressure at bottom of interval, atm Pt = pressure at top of interval, atm Gdh = gradient of heavier fluid determined from upper interval, atm./m Gdl = gradient of lighter fluid determined from upper interval, atm./m H = distance between points of measurement of Pt and Pb GdI - GdhWhere ------------------------------ (4) 3
- 5. Pressure at the perforations It is not possible to measure pressure at perforations. It can be reduced using following formula: Pp = Pb + ------------- ( Hp Hb ) Pp = pressure against the top of perforations, atm Pb = pressure at lowest depth of the survey, atm Pb-1 = pressure at the next lower depth of the survey, atm Hb = depth of the lowest measured pressure Pb , m Hp = depth of perforations, m Hb-1 = depth of Pb-1 ,m Pb - Pb-1 Hb - Hb-1 Where ------------------------------ (5) 4
- 6. Average gradient of the liquid column Depth of the base of liquid column Average gradient of the gas column Depth of the base of gas column Sub surface pressure Surface tubing or casing pressure Average Reservoir Pressure (approximation) Static Bottom hole pressure could not be determined for all the wells. Some wells representing whole of the reservoir are selected and static pressures are taken. These are reduced to common datum. Sub surface pressure can also be determined by approximation formula given as: = + x + x 5
- 7. Averaging method includes: (1)Well average pressure = (2)Areal average pressure= (3)Volumetric average pressure=--------- Average Reservoir Pressure pi n n 0 n 0 裡 ----------- 0 0 0 ---------- 裡 裡 裡 pi Ai n Ai n n 裡 pi Aihi Aihi
- 8. Isobaric maps Isobaric maps show by contours the reservoir pressure of a field or pool. These are especially useful in showing the areas of decline in pressure in a particular reservoir. Sets of isobar maps at regular intervals show the progressive change in pressure. 6 PRESSURE REPRESENTATION
- 10. Pressure gradients Gas zone: Less than 0.023 atm /m (0.1 psi /ft) Oil zone : Varies from 0.064 to 0.0874 atm /m (0.28 to 0.38 psi/ft) Water zone : Varies from 0.099 to 0.106 atm /m (0.433 to 0.465 psi/ft) I M P
- 11. EXAMPLE The table given below may be used to compute: (a) Pressure gradients. (b) Depth of gas- oil and gas water inter face (c) Pressures at GOC and WOC Depth m 0 200 400 600 800 1000 1200 1400 Press. atm 51.6 55.2 59.2 63.8 81.4 99.0 119.0 140.0 GIVEN : Depths of top perforation : 1150 m GOC : 740 m WOC : 1170 m
- 12. Measured value Calculated value Depth (m) Pressure (atm) P (atm) Gradient (atm/m) 0 51.6 - - 200 55.2 3.6 0.018 400 59.2 4.0 0.020 600 63.8 4.6 0.023 800 81.4 17.6 0.088 1000 99.0 17.6 0.088 1200 119.0 20.0 0.100 1400 140.0 21.0 0.105 1: Computation of pressure gradients
- 13. Depth of gas oil interface Refer 際際滷 5 Hc = H1 + ------------------------------------ = 400 +-------------------------------- =400 +----------------------- =400 +200=600m (Pb - Pt) - Gdh H 0.088 0.023 ------------------------------ (4) (63.8- 59.2)-0.088x200 Gdl - Gdh (4.6 - 17.6) -0.065 Hc = Depth to interface, m H1 = Depth to top of interval, in which interval occurs, m Pb = pressure at bottom of interval, atm Pt = pressure at top of interval, atm Gdh = gradient of heavier fluid determined from upper interval, atm./m Gdl = gradient of lighter fluid determined from upper interval, atm./m H + distance between points of measurement of Pt and Pb Where
- 14. Refer 際際滷 5 (99.0- 81.4)- 0.105x200 0.088 0.105 (17.6 - 21.0) 0.17 Depth of oil-water interface Hc = 800 + ------------------------------------ = 800 +--------------------------- = 800 +200=1000m
- 15. Pressure of the top of perforation can also be determined using equation 1 Pp = 99 +0.088(1150 1000)=99+0.088 x 150 =99+13.2=112.2 Pressure at GOC: PGOC=112 .2 + 0.088 (740 -1150) = 112.2 + 0.088(-410)=112.2-36.8 =76.12 atm Pressure at WOC: PWOC=112 .2 + 0.088 (1170 -1150) = 112.2 + 0.088(20)=112.2+1.76 =113.96atm