Gel Placement in Production Wells

Abstract

Straightforward applications of fractional-flow theory and material-balance calculations demonstrate that, if zones are not isolated during gel placement in production wells, gelant can penetrate significantly into all open zones, not just those with high water saturations. Unless oil saturations in the oil-productive zones are extremely high, oil productivity will be damaged even if the gel reduces water permeability without affecting oil permeability. Also, in field applications, capillary pressure will not prevent gelant penetration into oil-productive zones. An explanation is provided for the occurrence of successful applications of gels in fractured wells produced by bottomwater drive. With the right properties, gels could significantly increase the critical rate for water influx in fractured wells. Introduction Coping with excess water production is always a challenging task for field operators. The cost of handling and disposing produced water can significantly shorten the economic producing life of a well. The hydrostatic pressure created by high fluid levels in the well also is detrimental to oil production. The two major sources of excess water production are coning and channeling. Water coning is common when a reservoir is produced by a bottomwaterdrive mechanism. Fractures and high-permeability streaks are the common causes of premature water breakthrough during waterfloods. Polymer gels have been applied to many wells to reduce excess water production without adversely affecting oil production. l-6 Moffitt reported that polymer gels are particularly effective in suppressing water coning. In many cases, however, gel treatments have not been successful. Part of the reason for the sporadic success was problems with gel placement. During gel placement in production wells, much of the gel formulation will enter zones responsible for the excess water production. However, some of this fluid may enter and damage oil-productive strata. The objectives of this study are to model gel placement in production wells mathematically and to examine the potential effect of gelant invasion into oil-producing zones. Particular attention is paid to the importance of two phenomena. The first is hysteresis of oil/water relative permeability curves during the "pump-in, pump-out" sequence used during gel placement in production wells. The second phenomenon is that gels (or polymers) can reduce the relative permeability to water more than to oil. Sensitivity studies covering most known field and laboratory applications are discussed. In particular, we study permeability contrasts from 1 to 1000, oil/water viscosity ratios ranging from 0.1 to 100, endpoint water relative permeabilities ranging from 0.1 to 0.7, water saturations ranging from 0.2 to 0.8, and fractured and unfractured wells. Therefore, our conclusions should be applicable to most field applications of gels in production wells. Examples are provided to illustrate and contrast situations where gels are or are not expected to damage oil productivity. In these example...

Aperture-Tolerant, Chemical-Based Methods to Reduce Channeling

“…Fig. 4 and Column 9 of Table 1 [6][7][8][9][10]. Consequently, variability of clean up times may be manageable for wells within a given field.…”

“…[1][2][3][4][5] This disproportionate permeability reduction (or "relative permeability modification") is essential if polymers or gelants are placed in production wells without protecting hydrocarbon-productive zones. 6 With existing polymers, gels, and technology, disproportionate permeability reduction may have its greatest value when treating production wells that intersect a fracture or fracturelike features. [7][8][9] Nonetheless, many people are very interested in exploiting this property to reduce excess water production from unfractured wells (i.e., radial flow into porous rock or sand).…”

“…To avoid excessive losses in oil productivity when gelant is placed using unrestricted injection (i.e., no zone isolation), the gel must provide a residual resistance factor less than 2 in the oil zone. 6,13,17 Preferably, the gel should provide a residual resistance factor greater than 20 in the water zone (Fig. 1).…”

“…Depending on the magnitude of this effect, these polymers and gels can harm injection or production flow profiles in wells, even though the polymer or gelant penetrates significantly farther into the high-permeability rock. 6,13,17 Overcoming the Obstacles Variability. Variability of residual resistance factors was the first challenge mentioned above.…”

Conformance Improvement Using Gels

“…If this case applied, a gelant treatment would not be effective because near wellbore treatments cannot alter the pseudo-steady state fractional flow of a single producing zone. 43 Alternatively, a small fraction of the original net pay may have continued to produce nearly 100% fractional oil flow, while most of the net pay was watered out. This scenario could be amenable to successful treatment using gelants.…”