Oil Well Stimulation

Oil well stimulation plays a vital role in production operations. With oil prices at all-time highs, it is imperative from an oil company's perspective and the consumer's perspective that as much production as possible be safely extracted from the reservior.

Why? So, the oil company can realize the highest price per barrel, and the consumer can get more oil circulating in supply to balance demand. But then, we digress...

It has been said by many wise petroleum investors (oxymoron?) that the industry's saving grace is that their assets are in the ground! Once found, they argue, it is difficult to lose. Clearly, these investors weren't reservoir engineers as will be explained below.

Producing that oil isn't as simple as running the kitchen faucet and watching the basin fill-up (see oil production discussion) . Natural production tendencies for wells are for the oil production rates (and reservoir pressure) to be at its highest at initial production, and fall-off considerably as the well is produced. Typically, one finds oil rates declining as water production increases, driving up operations costs while revenue shrinks. This scenario continues until the well fails and/or becomes uneconomic to operate or repair.

The purpose of oil well stimulation, then, is to increase a well's productivity by restoring oil production to original rates less normal decline, or to boost production above normal predictions.

So, what is oil well stimulation?

Oil well stimulation is the general term describing a variety of operations performed on a well to improve its productivity.

Stimulation operations can be focused solely on the wellbore or on the reservoir; it can be conducted on old wells and new wells alike; and it can be designed for remedial purposes or for enhanced production. Its main two types of operations are matrix acidization and hydraulic fracturing.

Matrix acidization involves the placement of acid within the wellbore at rates and pressures designed to attack an impediment to production without fracturing or damaging the reservoir (typically, hydrofluoric acid is used for sandstone/silica-based problems, and hydrochloric acid or acetic acid is used for limestone/carbonate-based problems). Most matrix stimulation operations target up to a ten foot radius in the reservoir surrounding the wellbore.

Hydraulic fracturing, which includes acid fracturing, involves the injection of a variety of fluids and other materials into the well at rates that actually cause the cracking or fracturing of the reservoir formation. The variety of materials includes, amongst others: water, acid, special polymer gels, and sand. The fracturing of the reservoir rock and the subsequent filling of the fractured voids with sand ("proppant") or the creation of acid channels allows for an enhanced conduit to the wellbore from distances in excess of a hundred feet.

So, why do wells need oil well stimulation?

Hydraulic fracturing and acid fracturing in practically all types of formations and oil gravities, when done correctly, have been shown to increase well productivity above that projected in both new and old wells. From an economic standpoint, oil produced today is more valuable than oil produced in the future. Fracturing candidates may not necessarily "need" oil well stimulation, but the economics may show that such a treatment would pay=off.

To understand why remedial stimulation (matrix acidization) is necessary, you have to consider the conditions at work, deep down inside the reservoir...

Before the well is ever drilled, the untapped hydrocarbons sit in the uppermost portions of the reservoir (atop any present water) inside the tiny pore spaces, and in equilibrium at pressures and temperatures considerably different from surface conditions.

Once penetrated by a well, the original equilibrium condition (pressure, temperature, and chemistry) is permanently changed with the introduction of water or oil-based drilling fluids loaded with suspended clays, and the circulation of cement slurries. The interaction of the introduced fluids with those originally present within the reservoir, coupled with pressure and temperature changes can cause a variety of effects which, in turn, can plug the numerous odd-shaped pores causing formation damage. Some of the types of damage include: scale formation, clay swelling, fines migration, and organic deposition.

Petroleum engineers refer to the level of formation damage around the wellbore as skin effect. A numerical value is used to relate the level of formation damage. A positive skin factor reflects damage/impedance to normal well productivity, while a negative value reflects productivity enhancement.

Formation damage, however, is not limited to initial production operations. Remedial operations of all kinds from well killing to well stimulation itself, can cause formation damage. Nor is fines and scale generation limited to the reservoir. They can also develop in the wellbore in casing and tubulars, and be introduced from surface flowlines and incompatible injection fluids. These fines and precipitates can plug pores and pipe throughout an entire oil field.

In short, any operation throughout a well's life can cause formation damage and impede productivity.

So, how do you keep from damaging the formation while stimulating a well?

Oil field service companies offer chemical inhibitors useful for the full spectrum of well operations that can be injected into the well ahead of acidization or other such stimulation activities to reduce fines generation and organic deposition, or introduced in surface flowlines on a regular basis to prevent scale precipitation and build-up.

Operational techniques such as bringing on production or injection slowly after stimulation activities (to prevent damaging flow surges which could mobilize once immobile fines within the pores, plug perforations, or cause sand control problems downhole or at the surface), and routine maintenance and surveillance (like cleaning out process filter traps which can easily clog lines and cause the transfer of damaging suspended particles, or monitoring production decline to identify potential deviances before the problem is exacerbated).

Importantly though, a sound understanding of formation damage causes, and the inclusion of chemists/chemical engineers on the production team will lead to increased well productivity and life.

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