Case History Evaluation of RPMs on Conform Fracturing Applications
Actual worldwide oil production averages some 75 million barrels per day and, while estimates vary, this is associated with the production of 300 – 400 million barrels of water per day. These values of approximately 5 – 6 barrels of water for every barrel of oil are quite conservative. In some areas around the world, fields remain on production when the ratio is as high as 50 to 1.
Water production causes several problems to oil wells such as scaling, fines migration or sandface failure, corrosion of tubular, and kills wells by hydrostatic loading, amongst other things. Thus, while water production is an inevitable consequence of oil production, it is usually desirable to defer its onset, or its rise, for as long as possible.
Numerous strategies, both mechanical and chemical, have been employed over the years in attempts to achieve this. Simple shut-off techniques using cement, mechanical plugs and cross-linked gels have been widely used. Exotic materials such as DPR (disproportionate permeability reducers) and the new generation of relative permeability modifiers (RPM) have been applied in matrix treatments with varying degrees of success. Most recently, Conformance Fracturing operations have increased substantially in mature fields as the synergistic effect obtained by adding a RPM to a fracturing fluid have produced increased oil production with reduced water cut in one step, consequently eliminating the cost of additional water shut off treatment later on.
This paper is an evaluation of various RPM materials commonly used on Conformance Fracturing treatments performed in the northeast of Brazil and other South American countries, rather than the usual laboratory testing methods and theoretical estimations. The paper also describes the technical design and operational methodology to treat single zone to laminated reservoirs with different mobility ratios. We believe conformance fracture techniques could significantly impact the development strategies of many fields worldwide.
Water cuts in oil producing wells continue increasing as time passes and oil fields mature. The source of the water can be either formation or injected water used for reservoir pressure maintenance. Limited reservoir thickness, excessive pressure drawdown, high water/oil mobility ratios and reservoir heterogeneities can cause water channeling through high permeability streaks, hairline fractures and/or near-wellbore water coning early in the well's productive life. However, mobility is not the only factor. Wettability effects caused by heavy oil deposition, or capillarity effects caused by depletion such as trapped condensate, can also compound the problem.
Many reservoirs may not prove commercial without proper stimulation treatments. These may include dirty sandstones, lower permeability layered formations in water drive reservoirs and/or with nearby water zones.
Sandstone acidising is usually a damage removal exercise and, at best, can produce only about 1.4 folds production increase in undamaged formations.
Radial, matrix conformance treatments in these types of wells are not a commercial option due to the treatment cost and limited production potential, so many of these zones are never produced around the world and a substantial volume of oil is left behind.
So far, we have discussed matrix-type treatments, since hydraulic fracturing stimulation treatments have traditionally been designed to avoid water zones, at all costs. A typical result has been to end up with a water producer after the fracturing attempt, because the high-conductivity propped fracture reached the wet zone (analogous to having a high vertical permeability), and also to the unfavorable water/oil mobility ratio.
As we have not yet mastered fracturing design or its execution to effectively control and restrict fracture height growth, the fracture stimulation technique has not been a valid option for these marginal reservoirs and again, a substantial volume of oil has been left behind.
Since both treatments (conformance and fracturing) are required, a unique and cost effective solution will be to perform them simultaneously. In this scenario the only cost associated to the conformance treatment would be the materials themselves.GO TO paper