No abstract

The application of production chemicals downhole via dedicated injection lines or by means of gas lift systems, whilst not new, is becoming more commonplace in the oil and gas industry. The corrosion characteristics of injected chemical formulations in such systems can be complex and require detailed testing to understand the behaviour and assess the potential corrosion risk of chemical formulations under the extreme conditions of temperature and pressure that exist within either of these injection systems.In the presence of a fast flowing stream of hot gas, the more volatile components can be stripped from the formulated product, resulting in separate condensed vapours and chemical residues. Similarly, low-pressure vapour pockets formed within downhole chemical injection systems due to hydrostatic pressure fluctuations may cause some components of the injected chemical to be evaporated. Materials compatibility tests are commonly conducted using samples of "as supplied" production chemicals However, there is also a risk that different material compatibility/corrosivity characteristics can be induced by the condensed vapours or stripped residues of the production chemicals when compared to the originally applied chemical formulation. Laboratory testing is not commonly conducted to assess this behaviour.Samples have been prepared and corrosion tests conducted on collected vapours and residues from production chemicals. Distillations were conducted on the neat chemicals to obtain vapours and residues, and corrosion rates were measured on a range of different metallic specimens by conventional electrochemical or coupon weight loss methods. The results of these separated phase corrosion tests were then compared with the corrosion measurements on the original formulations.The test programme demonstrated that for some of the formulations tested, significantly higher corrosion rates were measured for either the condensed vapours or the distillation residues when compared to the original formulation.This work highlights the need to carefully consider the possible range of temperature/pressure conditions and gas/liquid phase interactions that will be experienced by injected chemicals within the chemical delivery system. Specifically, there is a need to assess the implications of solvent stripping/evaporation of volatile components. Chemicals that form more corrosive condensed vapours or residues may need to be avoided to prevent equipment failure or a loss of integrity. It is not adequate to simply test the 'stability' of a chemical at elevated temperatures, ambient pressure and static conditions; the separation effects within the gas/liquid system must also be considered.

The use of inflow-control valves (ICVs) and inflow-control devices (ICDs) to improve production rates in horizontal wells has become increasingly common in recent years. These devices have small apertures resulting in high, local high flow rates which results in regimes of very high shear and turbulence, potentially resulting in materials failure due to accelerated corrosion rates, erosion and potentially erosion-corrosion.To meet the challenge of testing accelerated corrosion rates, various laboratory methods have been developed to study the effects of increasing shear on corrosion. Common test methods such as rotatingcylinder electrode (RCE) tests can provide useful data at moderate shear stresses (up to 100 Pa) and ambient pressures, while rotating-cage autoclave tests (RCA) and rotating-cylinder electrochemical autoclave (RCEA) tests allow moderate shear tests to be conducted at elevated temperatures and pressures. However, achieving the very high wall shear stresses seen with certain oilfield jewellery, such as ICVs and ICDs, is significantly more challenging.In contrast, jet impingement (JI) methods have enabled materials testing at up to 10,000 Pa, and by coupling these with the ability to conduct these tests under increasingly higher pressures and temperatures, very-high-shear systems can be tested under conditions closely matched to those in the field. The approaches developed in our laboratories, which use both weight loss and electrochemical corrosion measurements, have also proved to be robust even in extremely corrosive environments, such as in the presence of stimulation acids (both uninhibited and inhibited) and over extended exposure times (Ͼ 7 days).These jet-impingement test methods have enabled enhanced understanding of the susceptibility of various materials to corrosion and erosion under extremely high shear conditions, and how effectively (or not) film-forming corrosion inhibitors perform. The application of these advanced laboratory techniques is currently playing a vital role in evaluating suitable methods for preventing corrosion under very challenging conditions before deployment in the field.

While downhole chemical squeeze treatments are common practice for deploying scale inhibitors to protect wellbores and downhole production tubulars from inorganic deposits, their adoption for corrosion inhibitor treatments (CISQ) is much less commonplace. Historically they have often not been considered an option due to either anticipated poor lifetimes or the risk of formation damage in the form of emulsion blocks, wettability changes or relative permeability modification. However, in a number of carbonate reservoirs producing from different regions, corrosion inhibitor squeeze treatments have been shown to provide corrosion protection for downhole production tubulars when no alternative downhole chemical deployment technique is available. This paper will present a technical review of the potential benefits of CISQ treatments. Different inhibitor retention/release mechanisms in sandstone and fractured carbonate reservoirs will be discussed, including classic adsorption/desorption and chemical imbibition/diffusion, as will the impact of these mechanisms and oil/water partitioning behaviour on squeeze lifetimes. Another factor limiting adoption of CISQ treatments is the misconception that residual assay techniques may be inadequate, and we demonstrate that this is not necessarily the case.In addition to providing a mechanistic interpretation of the available retention and release mechanisms, the results of laboratory core flood studies will illustrate the different retention mechanisms and offer the potential to achieve considerably different lifetimes with different types of corrosion inhibitors. Field examples will also be cited in support of this. In these cases, the accuracy of the measured residual corrosion inhibitor return concentrations were confirmed by conventional electrochemical corrosion inhibitor performance measurements.

Current scale risk analysis focuses on thermodynamic calculations to determine the risk of scale, ignoring system kinetics and the impact of flow regimes on scale precipitation from mildly oversaturated systems. It is however recognised that flow regimes affect scale precipitation. Surface growth is influenced by mass transport and diffusion which are susceptible to shear stress and turbulence. Little work has been reported which examine these factors under conditions that can be readily tuned to match field production conditions. Scale inhibitor evaluation exercises therefore often rely on conventional low shear/static or laminar flow conditions which have been demonstrated in many papers to be largely inadequate for mildly oversaturated systems. This work addresses this concept and focuses on scale deposition and growth at metal surfaces as well as bulk (liquid phase) nucleation and growth in mildly oversaturated brines as a function of increasing shear. A series of controlled experiments have been conducted under “mildly oversaturated” conditions to examine the effect of; no shear conventional “static” tests, moderate shear mixed statics and much higher shear regimes including rotating cage and jet impingement approaches with calculated shear stresses up to 500 Pa and higher. This builds on previous work published by the authors in this area1 and further illustrates the importance of conducting tests at field representative shear conditions. Since shear and turbulence have a governing effect on the critical scaling tendency (the level of oversaturation below which brines remain stable under normal production conditions) the ability to correlate between shear and the propensity for scaling in mildly oversaturated systems is critically important in determining the risk of scale at different locations in the production stream. New test methods have been validated which allow the impact of shear and turbulence to be observed under conditions more representative of production conditions. These methodologies lead to scaling in mildly oversaturated brine systems without having to adjust brine chemistry or otherwise increase the scaling regime, i.e. by adjusting the flow regime to reproduce the shear expected at critical locations in the production system. Improved methodologies are therefore presented which allow more appropriate scale inhibitor qualification, taking into account the impact of shear and turbulence under field representative conditions. The work shows that this is critically important for mildly oversaturated conditions.