Rock Physics and 4D Seismic for Improved Oil Recovery

1997 SEG Spring Distinguished Lecture

The worldwide volume of already discovered but unrecovered oil is by now enormous – on the order of (according to some) the oil that still remains to be discovered. On average, only 25% of discovered oil is being recovered in the world, whereas 65% to 75% is considered recoverable. Consequently, improved oil recovery, especially in large oil fields, is becoming a major target for oil producers worldwide.

Two major reasons for the wide spread poor oil recovery are:

1. Wellbore damage, caused by drilling, completion, and production: and
2. Subsurface reservoir heterogeneities of the rock properties that control production and fluid flow: permeability, porosity, rock and fluid compressibility, fluid saturation, fluid viscosity, etc.

In this lecture, I will show how rock physics can enable geophysicists to provide means to significantly help improve recovery, in both new and old fields. The reason for this optimism is that seismic wave velocities and attenuation in most sedimentary rocks are sensitive in various ways to the state of porous rocks, the fluids in their pores, and the state of these pore fluids.

Because reservoir parameters generally vary from place to place in an oil field, or change with time during oil and gas production, they often lead to observable changes in measured seismic attributes in the volume subject to production. Consequently, it should be possible – provided we develop the appropriate technology and techniques – to delineate or map the distribution of subsurface fluids and monitor their movement with time from surface seismic and repeated surface seismic measurements, and from sonic and repeated sonic logs.

In the first part of my talk I will consider two questions:

a) What can be detected seismically and acoustically?

Here I will show experimental and theoretical evidence for the influence of the two main groups of reservoir rock parameters that influence the seismic response of rocks-velocity, impedance, amplitude and frequency content:

1. Mechanical parameters: pore pressure, stress, porosity, strength, porosity, amount of clay, stress;
2. Fluids: oil, brine, gas, drilling mud saturation and partial saturation, type of oil, amount of gas dissolved in oil, temperature, pore pressure, chemical composition.

b) How to apply these effects to improved recovery?

There are two approaches to the use of geophysics for improved recovery:

1. Characterization: Identify spatial patterns in the data and use forward modeling to interpret it;
2. Monitoring: Tracing of fluid flow, injection, pore pressure changes, and temperature changes from repeated seismic and log surveys.

The main advantage of characterization is that it applies before drilling or production, thus providing information early in the development of a field. The main difficulty of characterization is the inherent non uniqueness of the interpretation: because a variety of combinations of pore pressure, porosity, fluid saturation, and temperature can give rise to the same seismic signature it is sometimes difficult to tell what is in the subsurface.

The main advantage of monitoring is the almost unique interpretation of the time changes of the seismic, or sonic signature due to our preknowledge of the recovery process, the fluids involved, and pore pressure and their time changes, and the depth and approximate location of recovery. The main disadvantage of monitoring is that it can be used only after drilling and after production has begun. It appears therefore that monitoring will play in the near future a major role in improving recovery in older producing fields or fields whose production has declined significantly even though a lot of oil still remain - in principle to be recovered.

In the second part of my talk I will show example of some monitoring results to date:

1. Steam floods,
2. Water floods,
3. Co2 injection,

and discuss ongoing research related to future monitoring, e.g.

1. Repeated sonic log to track formation damage;
2. How to design a monitoring strategy before a new field goes into production.

Because the economics of recovery monitoring appear very encouraging, It IS conceivable that in the future many large but relatively unproductive fields will require a continuous monitoring treatment to maximize recovery. This prospect provides both a challenge as well as a great opportunity to geophysics and reservoir engineering in both industry and in academia. We will have to adapt existing seismic and logging methods and possibly develop new ones, and collaborate with production engineers in a way we never did before.