RESERVOIR DYNAMICS-I

The reservoir development is key to hydrocarbon exploration.

Primary & Secondary recovery

Primary recovery uses the using natural energy stored in the reservoir. Secondary recovery would imply adding some energy to the reservoir by injecting fluids such as water, gas etc so as to impart energy to the reservoir.

1.       Solution gas drive (Depletion drive)

Solution gas drive is the drive which occurs in a reservoir which contains no initial gas cap or underlying active aquifer to support pressure. The driving force is the expansion of oil and connate water plus any compaction drive. Once bubble point is reached the dissolved gas forms a secondary gas cap. This can be encouraged by reducing pressure at producing wells. The producing wells should be located as low dip possible so that gas can accumulate at the crest of the reservoir and assist further recovery.

The facilities are designed for plateau period for an oil field. A plateau period is period when optimum balance between producing oil and avoiding unfavorable displacement in the reservoir is reached.

The production rate is maintained until abandonment rate is reached. Abandonment rate is referred to as when cost of production is greater than the revenues from production. For depletion drive plateau period is short and decline is rapid. Water cut is low in depleting drive as there is little or no support from the aquifer. A typical RF from depleting drive is 5-30%. Secondary recovery techniques have to be adopted in order to boost production.

2.       Gas Cap drive

The initial condition required for gas cap drive is an initial gas cap. The high compressibility of gas provides drive energy for production. Well are initially positioned away from gas cap so that gas can accumulate. Also well are placed not too close to OWC in order to prevent water coning.

A typical RF for gas drive is in range 20-60%. The producing GOR increases as expanding gas cap approaches producing well.

Natural gas cap drive may be supplemented by reinjection of produced gas or any other source into the crest of the reservoir.

3.       Water Drive

Natural water drive occurs when underlying aquifer is large and water is able to flow into oil column. To identify the type of drive in a reservoir remains a major uncertainty during reservoir development. Hence it is necessary to see the response of the well over a period of time such as reservoir pressure, fluid contact and material balance techniques.

Wells are drilled up dip and water is injected low dip in the aquifers. This drive has a long plateau period and increase in water cut over period of time. Water cut may reach 90% in final part of the field life. Process engineers have to accordingly plan surface facility so as to handle large quantity of fluid. The RF is between 30-70%.

There may also be a case where there is combination of drives.