WATERFLOODING: GEOLOGICAL PERSPECTIVES OF RESERVOIR ENGINEERING

Based on a workshop sponsored by PTTC’s South Midcontinent Region and the Oklahoma Geological Survey on June 10-11, 1998, in Oklahoma City, OK.

BOTTOM LINE

Since the future value of a waterflood is based on the extrapolated price of oil, the economic analysis must accurately predict price trends if the project’s economic goals are to be met.

PROBLEM ADDRESSED

Prior to the onset of a waterflood, there must be an accurate evaluation and prediction of reservoir condition. This includes an analysis of the architecture, structure, presence and orientation of fractures, as well as reservoir limits, rock, and fluid property and pressure distributions.

KEY WORDS:

Waterflood, Gas-to-Oil Ratio, Reservoir Architecture, Production Analysis, Fractures

SPEAKERS

Kurt Rottmann, Consulting Geologist

With support from:
David Crutchfield, Consulting Engineer

TECHNOLOGY OVERVIEW

Since waterfloods are typically implemented in mature developed properties, it might be assumed that the reservoir is well defined, with few potential surprises. Experience indicates otherwise. However, operators who are aware of common pitfalls can avoid mistakes that will reduce recovery and profitability when implementing waterfloods.

The growth of a waterflood prospect, from conception to abandonment, is divided into two stages. The first stage, which is the topic of this workshop, deals with the initial analysis up to the development of the reservoir model. It is especially critical that engineers and geologists work together during this initial conceptual stage. The second stage, which is more engineering-oriented, deals with development of the reservoir model, implementation, and operation until project abandonment.

Typically, an operator’s first step in evaluating a waterflood candidate should be to assemble and evaluate preliminary reservoir data. Information about the gas: oil ratio (GOR) and water production provide key information about whether a project is an attractive candidate. Operators should examine GOR data, both on the basis of the well’s initial potential and cumulative production basis. Oil reservoirs exhibiting cumulative GORs that are less than 3000/1 are generally attractive candidates.

Wells with a GOR less than 3000/1 are generally attractive candidates. Gas reservoirs do not make favorable waterflood candidates because of the lack of sufficient oil to bank. The operator should, however, be aware that a well’s high GOR can be caused by comingling of fluids from different stratigraphic layers. Detailed stratigraphic correlation should be able to distinguish the types of fluids coming from various zones.

Operators should take special care looking for indications of a gas-oil contact or gas cap. If present, a gas cap will distort GOR data and its presence must be considered when designing a waterflood. It also is critical to note water-oil contacts, calculate fluid saturations, and identify sources of water production. In analyzing gas and water data, operators should be aware that they are not always accurately reported, so considerable judgment may be required.

A two-step analysis of reservoir data is used to evaluate a waterflood candidate. First, the operator reviews initial production potential, gas-to-oil ratios (GORs), amount and type of water production, pressure levels, and core information. If this information meets the requirements, a second, more thorough review of data of the reservoir’s fluids pressure, volume, and temperature (PVT) properties should be initiated in cooperation with the company’s engineering department. It also is important to understand the source of formation water within the reservoir and calculate its saturation. Formation water production can be caused by many processes, such as water saturation above irreducible water saturation, water entering the wellbore accidentally from another reservoir (e. g. hole in casing), water introduced mechanically (e. g. an injection well), or natural encroachment due to migration of the oil-water contact.

Production history should be analyzed, too. An analysis of primary recovery will determine not only if sufficient oil saturation remains for successful waterflooding but also if there are potential problems.

Determining net pay for a waterflood is important because the calculations affect original-oil-in-place (OOIP) values, reserve estimates, economics, and performance characteristics. Properties that can have an impact on net pay include porosity-permeability correlations, average porosity and thickness, impermeable parts of the reservoir, and fluid contacts. Mapping net pay may be the primary contribution of geologists to evaluating waterflood candidates.

Although it’s often the least-evaluated aspect of a waterflood project, the reservoir’s structure can have major influences, such as those caused by faulting (and fracture); degree of dip; and determination of overall homoclinal, anticlinal, or synclinal nature of the reservoir.

The proper techniques for core analysis begin even before drilling and continue through laboratory examination. Cores can be used to significantly increase data available about lithology, depositional environment, diagenesis, and rock type. Core samples are required to define pore geometry changes with depth, areal distribution, permeability, and porosity.

But fractures can be a positive benefit for wells under primary production because of increased permeability accessibility. This holds true whether the fractures are natural or induced, and whether they are produced from stress caused by local, regional or basinal factors.

Fractures also can be a detriment to hydrocarbon production because they may connect to downdip water. Because fracture permeability is often several orders of magnitude higher than the matrix permeability, injected water will generally travel through fractures. This can be disastrous for a waterflood if water is injected into a fracture that is in communication with a producing well.

Economic Implications:
Economic analysis, with all its incorporated data, predicts the success of the waterflood. As a result, neither the geologist nor the engineer alone is responsible for the prediction. A realistic price structure for the economic evaluation is often the deciding factor for economic success of waterfloods.

Long-range price trends are important, since the future value of a waterflood project is based on an extrapolated price of oil. Because the future price of oil is unknown, project economic analysis should incorporate a conservative price structure. Further, prices must be adjusted, taking inflation into account, as part of the economic analysis.

CONNECTIONS:

Kurt Rottmann, Consulting Geologist
7113 Northwest 30th St. Bethany, OK 73008
Phone 405-491-0333

David Crutchfield, Consulting Engineer
8120 NW 78th Terrace Oklahoma City, OK 73132
Phone 405-721-9216

For information on PTTC’s South Midcontinent Region and its activities contact:
Charles Mankin, Director, Oklahoma Geological Survey
100 E. Boyd St., Room N131, Norman, OK 73019-0628
P405-325-3031, Fax 405-325-7069, E-mail cjmankin@ou.edu

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