Pressure Regimes and Fluid Contacts

Abstract

The pressure of a reservoir is the key driving force for the primary recovery of hydrocarbon. It gives an indication of the amount of hydrocarbon remaining in the reservoir at any given time. Three types of pressure regime is encountered during the drilling process; which are the normal, abnormal and the subnormal pressure. During production of the hydrocarbon, as soon as the well is open to flow, the reservoir pressure starts declining, depending on the drive mechanism of the reservoir and at any point in time, the average value of pressure is estimated. In this chapter, solved example questions on determination of fluid contacts from pressure survey data and averaging of reservoir pressures are presented. In addition, the position of the reservoir fluid contact is not static as the fluid is produced. At first, the position of the fluid contacts are first determined within control wells and then extrapolated to other parts of the field. Once initial fluid contact elevations in control wells are determined, the contacts in other parts of the reservoir can be estimated. Hence, the various methods of estimating reservoir fluid contact are presented with solved example questions.

Exercises

Ex 8.1:

An exploratory well penetrates a reservoir near the top of the oil column. Logs run in the well clearly located the gas-oil contact at 5200 ft also DST test conducted on this well and sample analysis of the fluid sample collected from the same well gave reservoir pressure of 2402 psia at 5250 ft and oil gradient of 0.35 psi/ft the depth of the oil-water contact is uncertain because it could not be confirmed by logs.

• Determine the probable oil-water contact

• What is the pressure at the crest of the reservoir?

Ex 8.2:

Calculate the average reservoir pressure at the Datum depth of 8750 ftss for the following fluid pressure gradients, given that the GOC and OWC are at 8700 ftss and 8800 ftss respectively:

$$ {\left(\frac{dP}{dD}\right)}_{gas}=0.08\ psi/ ft,\kern0.75em {\left(\frac{dP}{dD}\right)}_{oil}=0.269\ psi/ ft,\kern1em {\left(\frac{dP}{dD}\right)}_{water}=0.434\ psi/ ft $$

Well A = 3685 psig at 8690 ftss

Well B = 3716 psig at 8800 ftss

Well C = 3725 psig at 8820 ftss

Well D = 3689 psig at 8710 ftss

Well E = 3713 psig at 8790 ftss

Ex 8.3:

A well penetrates a reservoir near the top of a fluid column. The GOC has been detected by logs but the OWC. An oil sample was taken at 7890 ft TVD with a pore pressure of 3080 psig recorded. The field water gradient is 0.445 psi/ft, oil gradient is 0.347 psi/ft find the OWC.

Ex 8.4:

The result of an RFT tests conducted on an appraisal well in a field located in the Niger Delta region is presented in the table below. Determine the types of hydrocarbons present and find the fluid contact.

Depth TVD (ft)	Formation Pressure (psia)
11,200	4648
11,300	4656
11,450	4664
11,500	4672
11,600	4730
11,700	4745
11,820	4778
11,900	4810

Ex 8.5:

The result of an RFT tests conducted on an appraisal well in a field located in the Niger Delta region is presented in the table below. Determine the types of hydrocarbons present and find the fluid contacts

Depth TVD (ft)	Formation Pressure (psia)
11,762	5816
11,829	5821
11,847	5822
11,977	5859
12,011	5871
12,087	5898
12,141	5915
12,269	5973
12,278	5976
12,316	5994
12,345	6009

Ex 8.6:

A pressure survey was carried out on a well that penetrates through the gas zone in a reservoir at FUPRE. The result of test 1 recorded a pressure of 3830 psia at 9525 ft with fluid gradient of 0.352 psi/ft while test 2 at 9200 ft recorded a pressure of 3560 psia with fluid gradient of 0.118 psi/ft. Calculate:

• Estimate the fluid contacts (GOC & OWC ) in the reservoir

• During history match, it was observed that the fluid contacts given by the geologists were wrong which was traceable to wrong fluid gradient . After careful analysis, it was observed that the oil gradient is 0.341 psi/ft recomputed the fluid contacts and estimate the absolute relative error.

• The thickness of the oil column

• Calculate the pressures at GOC and OWC respectively

Hint: take the water gradient as 0.445 psi/ft and atmospheric pressure as 14.69 psia

Ex 8.7:

An exploratory well penetrates a reservoir near the top of the oil column. Logs run in the well clearly located the gas-oil contact at 5200 ft also DST test conducted on this well and sample analysis of the fluid sample collected from the same well gave reservoir pressure of 2402 psia at 5250 ft and oil gradient of 0.35 psi/ft the depth of the oil-water contact is uncertain because it could not be confirmed by logs.

• Determine the probable oil-water contact

• What is the pressure at the crest of the reservoir?