This paper describes the development of a PMPA scale inhibitor chemistry, for potential ‘squeeze’ application, to provide scale inhibition under severe Barium Sulphate scaling conditions. Phosphonate type inhibitor chemistries have been applied in ‘squeeze’ treatments as they have the desired adsorption characteristics to protect the near wellbore formation, perforations and production strings for a significant period of time (known as the ‘Squeeze’ Life). These chemistry types, however, are not the most efficient means of preventing scale deposition in some severe, low pH BaSO4 scaling situations. Polymeric inhibitor species, in particular PolyVinylSulphonates (PVS) and sulphonated co-polymers (VS-Co), are sometimes more suited to these types of conditions, but they do not typically posses the necessary retention characteristics to achieve the desired ‘squeeze’ lifetimes. This paper discusses the development of a new experimental PMPA chemistry which has demonstrated improved inhibition performance under harsh BaSO4 conditions, whilst retaining the adsorption and retention characteristics of earlier PMPA chemistries and could potentially result in considerably longer ‘squeeze’ lifetimes than those currently achieved with the polymeric inhibitors.
The formation of oilfield scale, in particular barium sulphate, is recognised as one of the major problems associated with oil and gas production1–4. One of the most commonly used methods to control this is by the treatment of the near wellbore formation with a chemical scale inhibitor, in what is described as a ‘squeeze’ treatment4–8. In this type of treatment the inhibitor chemical is injected or ‘squeezed’ into the rock formation. When the well is put back onto production, this material will usually return in the produced fluids at a level that is greater than that required to prevent scale deposition. This concentration is known as the Minimum Inhibitor Concentration, or MIC and is typically in the order of 1–15ppm of active inhibitor. The length of time that the concentration of inhibitor in the produced fluids exceeds this MIC is known as the ‘squeeze’ lifetime. This is usually defined in terms of cumulative barrels of water produced since the treatment and depending on the production rate of the well can equate to a time period of between 3 and 12 months, or in some cases longer.
Chemistries that are suitable for such an application must fulfill two criteria. Firstly, they must inhibit scale deposition at the sub-stoichiometric/threshold levels described above. Secondly, the material must be retained within the rock matrix. Retention, either by adsorption or precipitation, enables an inhibitor to return from the formation at levels above the MIC for a significant period of time. One such inhibitor type, which utilises the PhosphonoMethylated PolyAmine (PMPA) chemistry, has been found to possess extraordinary adsorption properties and, as a result, achieve significantly longer ‘squeeze’ lifetimes than a conventional phosphonate inhibitor chemistry (DETPMP - DiEthyleneTriamine PentaMethylenePhosphonate)9. The main drawback with some phosphonate type inhibitor chemistries, including PMPA, is that they are not always the most suitable types of chemistry to prevent Barium Sulphate deposition in some of the more severe scaling situations. Polymeric species, such as PolyVinylSulphonates (PVS) and sulphonated co-polymers (VS-Co), are more suited to the prevention of scale deposition in these types of conditions, however their retention characteristics are considerably less favourable in achieving suitable ‘squeeze’ lifetimes.