Bridging Adsorption of Cationic Polyacrylamides in Porous Media

Abstract

Recently, cationic polyacrylamides (CPAM) have been successfully applied in water shutoff treatments of oil and gas wells. These polymers adsorb strongly on reservoir rocks, building up an adsorbed layer of significant thickness. Moreover, under high rates, the coiled macromolecules stretch and can bridge large pore throats. This so-called bridging adsorption mechanism has been described previously for high-molecular-weight non-ionic polyacrylamides (PAM). CPAM rheology and retention behavior has been studied in unconsolidated SiC packs and in Berea sandstones in a permeability range 0.1–1 D with CPAM solutions having a cationicity between 0 and 50%. Due to the attraction between the positive charges carried by the polymer chain and the negative surface charges of the rock, both CPAM adsorption and bridging adsorption are higher than for PAM having the same molecular weight. A maximum adsorption is found around 10–15% cationicity. This maximum, observed both in SiC and in Berea, is due to the competition between adsorption energy and pore wall accessibility. Although the permeability to water drops considerably after CPAM bridging adsorption, the permeability to oil is remarkably preserved, which makes CPAM attractive for water shutoff applications. Introduction Operators of mature oil and gas fields are often faced with a high water production coming from a water source (aquifer) or resulting from water injection. Excessive water production generally causes economic and operational problems. It decreases oil production, results in large amounts of water that need to be disposed and gives extra costs related to oil/water separation, handling and lifting. Other problems include the increased tendency for the formation of emulsions, scale and corrosion. So a high water production decreases the economical lifetime of a well and there is a need to reduce it. Classically the following distinction is made regarding the treatments to be used to tackle the water problem.1 If water and hydrocarbon zones are clearly separated, a permanent barrier, which is placed in the water producing zone, should be applied (Treatment A). Cements, resins or strong gels can form these full-blocking systems. If hydrocarbon and water zones are not clearly distinguishable or there is a high level of crossflow between layers, the use of total shutoff is risky. In this case disproportionate permeability reducers (DPR's), usually polymer solutions or weak polymer gels should be applied. The aim is to reduce water flow selectively while not influencing oil flow (Treatment B).1,2 The working of DPR's is based on the fact that adsorption of hydrophilic polymers can strongly decrease the relative permeability to water while having little effect on the relative permeability to oil. Due to the increasing need for bullhead treatments (treatments without zonal isolation), oilfield operators have focused on self-selective systems (Treatment B). These systems can be bullheaded downhole, reducing selectively the permeability to water with respect to the permeability to oil or gas. Due to this property polymers or weak gels were thought to be magic products that could be used in all situations. The relatively low success rate of DPR bullhead treatments (literature reports around 40%) shows reality is less favorable. Hereunder we will discuss three reasons for this by using an example of a DPR treatment on a two-layer well with 1/10 permeability contrast (Figure 1). Here the high-permeability layer is swept first, either by an active aquifer or by water injection. The low-permeability layer is still producing at high oil cut, although water production from the high-permeability layer is overtaking its oil production.

Hydrodynamic thickness of petroleum oil adsorbed layers in the pores of reservoir rocks

“…The skin formation in the pores of reservoir rocks by the injection of polymer solutions deals with polymer bridging adsorption [5][6][7]. Many researchers have investigated the measurements of polymer layers thickness adsorbed on solid surfaces.…”

Flow characteristics of three enhanced oil recovery polymers in porous media

“…Subsequently, the average thickness of the hydrodynamic polymer layer was determined using eq. where, e is the average hydrodynamic polymer layer thickness (μm), r is the average pore radius (μm) for brine flow which can be calculated using eq.…”

Effect of polymer on gas flow behavior in microfractures of unconventional gas reservoirs

“…The bridging absorption that differs radically from static absorption is presented when polymer coiling chains are adsorbed and form a bridge over the entrance of flow path. This mechanism could be enhanced when increasing superficial polymer velocity and the corresponding shear rate (Zitha and Botermans, 1998;Denys et al, 2001). Therefore, the bridging effect of polymer improved and more polymers were stuck within the narrower fracture.…”