The excessive production of water associated with gas can stop the development of a deposit. EniTecnologie and Divisione Agip have found a way to revive production at seemingly exhausted wells.
Note: originally published by E.N.I’s Ecos on 1999.
Oil companies drill and operate wells in order to produce petroleum hydrocarbons-oil or gas. While this is self-evident, it is less-well known that water always accompanies the hydrocarbon to the surface.
Early in the life of a well the amount of water produced may be very low - a few percent of total fluids - but, as the deposit is produced, the amount of water increases inexorably.
To understand why this is, we need to look "down-hole" for a minute. In the first place, hydrocarbons share the pore space in the porous reservoir rock with water (after all, on a geologic time scale, oil or gas is the "latecomer" to the reservoir, which is originally filled with water).
In most cases, this water is mobile and flows into the wellbore with the oil or gas. Further, as the amount of hydrocarbon present is reduced by production, water normally makes up an increasingly large portion of the fluids produced. In other cases, fractures or higher permeability zones in the rock may connect the well to an aquifer underlying the hydrocarbon-rich zone, or otherwise serve as "highways" causing the water to arrive in unacceptably large amounts, even early in the well's lifetime.
Ultimately, the economic limit for producing the hydrocarbon from a single well - or an entire reservoir - is determined by the cost of lifting the water to surface, separating it from the hydrocarbon, and disposing of it according to good environmental practice.
In favorable cases, oil production may still be profitable with a "water cut" (ie., the percentage of water in the total fluids produced) of 95 percent or higher (ie., nineteen barrels of water per barrel of oil). In other cases, it may cease to be economically viable at a much lower water cut.
In any event, when production from a well or reservoir must be stopped because of the excessive costs of managing water, the remaining hydrocarbon is in effect abandoned - perhaps forever.
Given this scenario, it is not surprising that oil companies devote considerable effort and resources to placing their wells and producing them in such a way that water production and costs are kept to a minimum. Nevertheless, a number of factors can intervene so that some wells (even in very productive reservoirs) produce unacceptably high levels of water, even when a large amount of recoverable oil remains in the reservoir.
When excessive water production presents itself as a problem, the well engineer has several technical "cards" he can play. These include mechanical isolation of the water producing zone, drilling a new well off the main well-bore to increase the distance from an underlying aquifer, etc. Often, however, these operations may not be technically feasible or economically justified by the reserves that remain in the reservoir.
Chemists in the oil industry have worked intensively over the past several years to place new, low-cost tools for reducing water production in the well engineers' hands.
The goal of this research, in which EniTecnologie and Eni Agip Division have played a leading role, is to identify chemical systems that can be pumped down the well-bore and into the reservoir rock in order to block water before it ever enters the well.
This article will describe one of the systems developed which shows high promise for blocking water production in gas wells in Italy - a strategically important asset in Eni's hydrocarbon portfolio.
Shutting off water in Italian gas fields:
Gas reservoirs in Italy, both on-shore and in the Adriatic Sea, are characterized by complex geology in which thin gas-containing sands alternate with impermeable clay layers.
Typically, "bundles" of layers a few meters in height are placed into production together. If one of the layers is more permeable than the others, water may soon arrive at the well from a nearby aquifer.
Even a small amount of water can entirely block gas production, if the reservoir pressure is insufficient to push gas to the surface through the column of water standing in the wellbore and into the gas pipeline. If he finds himself in this situation, the well engineer is forced either to intervene or to abandon these layers - together with the gas they still contain.
The problem with any type of well intervention aimed at stopping the water and leaving the gas-containing layers to produce the rest of their reserves, is that it is extremely difficult to identify which sub-layer is producing the water (it may be only a few tens of centimeters in height) and even harder to isolate it mechanically.
Researchers in EniTecnologie, working closely with Eni Agip Division's technical staff in San Donato, have developed a deceptively simple chemical treatment that essentially "seeks out" the water producing levels and shuts them off-without affecting gas production from the other layers.
This chemical treatment employs special non-toxic, water-soluble polymers (polymers are "macro" molecules prepared by "stringing together" a very high number of smaller repeating units called monomers).
These polymers were identified by means of extensive laboratory studies carried out with samples of sand and rock recovered from Italian reservoirs, and a coherent theoretical explanation has been developed to account for their action.
They are characterized by a very strong tendency to adsorb onto the reservoir rock, and by the ability of the adsorbed polymer to block selectively the flow of water. In a typical field treatment, a dilute solution of the polymer in brine is pumped into the reservoir where it invades all of the producing sand layers to a depth of a few meters around the well-bore.
The beauty of this treatment is that the adsorbed polymer blocks flow only in those layers producing water, leaving flow unhampered in the layers still producing gas.
This new technology has demonstrated its value in a recent application in the Carrera field in the Province of Foggia in Southern Italy. The well in question, Carrera-1, had a very short productive lifetime - it went on-stream in June, 1988 and closed after only 10 months - because excessive water production halted the flow of gas to the surface.
Having identified this well as an appropriate candidate for treatment, a two-day intervention with the custom - designed chemical "pill" was conducted by the EniTecnologie/Eni Agip Division team working closely with technical staff from the Ortona District and service suppliers.
The graphic display of the treatment results speaks for itself water production dropped approximately three - fold, and gas production-not seen for this well since 1989 - stabilized at over 20,000 standard cubic meters per day for eight months.
The treatment thus was not only a clear technical success, but an economic success as well: the cost of the treatment was recovered within the first two months of gas production, while the well continued to produce for an additional six months. When gas production from the well finally stopped again, it was for the "right" reason: the gas-containing layers had been fully exploited.
What is next for this new chemical technology? The successful field test has demonstrated its value in a typical, low temperature Italian gas reservoir, so an important objective is fully to exploit its potential in Italy.
The on-going R&D effort now is focused on the challenge of extending this treatment to other important targets - higher temperature reservoirs, reservoirs containing oil rather than gas, and to other reservoir mineralogies.
Of course, this chemical "pill", like any true medicine, is not a cure-all for water production. Rather, it effectively remedies some specific types of well "illnesses", as the case of Carrera-1 illustrates.
By Thomas P. Lockhart, Diego Giacca
© 2000 Mena Report (www.menareport.com)
