The deposition of scale in the near-well formation and production string can result in significant decrease in well capacity. Correctly designed scale inhibitor squeezes can successfully prevent scale deposition and the corresponding prevent scale deposition and the corresponding productivity declines. However, inadequate productivity declines. However, inadequate knowledge of the chemistry of the scale inhibitor and its interactions with the formation can yield ineffective squeezes. Inappropriately designed squeezes may actually damage the formation.
This paper addresses the chemistry of phosphonate scale inhibitors and the application of phosphonate scale inhibitors and the application of that chemistry in inhibitor selection and squeeze design. Screening/design criteria discussed include scale inhibition, thermal stability, corrosivity, scale dissolving capability, solubility in the reservoir brine, compatibility with multivalent cations, and adsorption/desorption on the reservoir rock. These criteria are illustrated with the results of an inhibitor screening study for the Prudhoe Bay field. Laboratory and field results illustrate how inhibitor chemistry can be used to optimize squeeze design.
During the production history of a well, productivity declines are often experienced which productivity declines are often experienced which are substantially greater than those predicted from reservoir studies. These productivity losses are frequently the result of carbonate and sulfate scale deposition in the production string or the near-wellbore formation. Scale deposition results when the chemical equilibrium of the formation brine is perturbed. Common perturbations are the pressure and temperature reductions accompanying pressure and temperature reductions accompanying production and the injection of incompatible waters production and the injection of incompatible waters during drilling, completion, and waterflooding. Regardless of their source, prevention and removal of scale deposits have been problems plaguing the oil industry since its earliest days.
Removal of the scale will increase productivity, yet often this removal is not easily productivity, yet often this removal is not easily or economically accomplished. Scale in the perforations and near-wellbore formation is not perforations and near-wellbore formation is not easily contacted by chemical treatments and many sulfate scales (especially BaSO4) can only be removed mechanically. Moreover, after removal, productivity decline reoccurs as new scale is productivity decline reoccurs as new scale is formed. Thus, the most effective treatment is the prevention of scale formation. This can be prevention of scale formation. This can be accomplished by changing the operating conditions to remove/dampen the scale forming perturbations. However, this is generally not practical. An alternative is to use scale inhibitors in the near-wellbore matrix to inhibit the formation of scale.
Scale inhibitors function at concentrations significantly below the levels required to sequester or chelate the scaling cations. The molar ratio of precipitate held in solution to inhibitor is typically on the order of 10,000:1. It has been postulated that scale inhibitors prevent, slow, or postulated that scale inhibitors prevent, slow, or distort crystal growth by blocking growth sites. It is also believed that the inhibitors prevent the adhesion of scale to metal surfaces in prevent the adhesion of scale to metal surfaces in some unknown manner Regardless of the inhibition mechanism, scale inhibitors must be present during scale nucleation in order to function effectively.
Numerous chemicals have been suggested as effective inhibitors but only four classes of compounds have been widely applied in the oilfield: polyphosphates, phosphonates, phosphate esters, and polyphosphates, phosphonates, phosphate esters, and polyacrylates/polyacrylamides. Typical structures polyacrylates/polyacrylamides. Typical structures for these four compound classes are given in Fig. 1.
If scale damage occurs in the near-wellbore formation and perforations, a source of inhibitor must be present in the reservoir. Frequently, scale inhibitors are pumped (" squeezed") into the formation where they adsorb on the reservoir rock. When a treated well is brought back onto production the inhibitor desorbs from the rock matrix production the inhibitor desorbs from the rock matrix into the produced brine.