No abstract

Hydrochloric acid (HCl) is commonly used to remove wellbore damage and to enhance near-wellbore formation permeability in carbonate formations. Although in most cases the spent acid is recovered rapidly during well flow back after the treatment, in heterogeneous reservoirs some of the spent acid can remain trapped for long periods becoming fully saturated with CaCO 3 under downhole conditions. These reservoir-saturated spent acids then lead to scaling when eventually unloaded into the production system. This paper describes a modelling and laboratory study to replicate the system, to allow selection of inhibitors which are effective against carbonate scales in spent acid solutions containing extremely high levels of Ca 2+ /Mg 2+ , and also remain stable at elevated temperatures in the spent acids. The work also examined chemical retention and release via coreflood testing followed by field application modelling to select effective scale inhibitors (SI) which possess poor retention properties on the carbonate substrate thereby remaining present in the "spent acid". One significant challenge associated with this study was the ability to reproduce the mildly oversaturated field scaling environment in the laboratory. When using these fully saturated (with respect to CaCO 3 ) partially acidic brines, very small changes in the brine chemistry or preparation procedures had a significant impact on scaling. A detailed evaluation of brine preparation, stabilisation and buffering was therefore required prior to evaluating generic scale inhibitors for performance under these extreme conditions leading to selection of appropriate species. The second stage of the work then involved core test procedures to determine those chemicals that offered minimal retention properties on the reservoir along with performance. This paper will present the field scaling challenges observed due to unloading of trapped spent acids; describe challenges faced within the laboratory in reproducing these conditions and present results from generic chemical types which are effective at preventing scale under these extreme conditions. From the shortlisted products further results are presented demonstrating those which offer poor retention, which is generally the opposite of what is required for conventional scale inhibitor squeeze treatments, allowing selection for upcoming field trials in the selected field system.

The ability to optimise the use of carbon steel in sour service presents man y economic advantages, including minimizing the use of expensive corr osion resistant alloys, enabling optimised large bore completions, and eliminating addit ional offshore facilities. An integ rated approach to corrosion modeling and testing has been d eveloped to reliably extend the ap plication of carbon steel. Key elements of this integrated approach include: (1) ri gorously establish tubular environmental conditions, (2) accurately simulate environmental conditions (acid gas concentratio ns (CO 2 /H 2 S), water c ut, and water composition) in the laboratory, (3) mathematically extrapolate laboratory weight loss and pitting to predict tubing life, and (4) validate tubing life predictions with field caliper data. This paper describes the application of the inte grated approach to evaluate L80 tu bular corrosion and predict tubing life for a range of a cid gas (CO 2 /H 2 S) concentrations. The pape r also discusses the corrosion mechanisms and validation of predicted corrosion with field caliper data.

This paper discusses an approach used to assess liquid film erosion/corrosion effects in the tubing strings of sour, high-rate, wet gas producers. This was done as an alternative to API RP 14E, which utilises an empirical erosional velocity factor "C" to estimate maximum velocity limits to minimise the potential for tubing metal loss from erosional effects.Many RasGas wells are completed with a full L-80 carbon steel or a combination L-80/Corrosion Resistant Alloy (CRA) production string. Once on production, a thin iron sulfide scale develops on the tubing wall significantly retarding the rate of metal loss due to internal corrosion. However, shear stresses generated from the condensate/water film flowing along the tubing wall could potentially remove this protective iron sulfide coating and expose fresh metal to much higher corrosion rates. This paper describes the approach adopted to assess the magnitude of shear stress created across a range of flow conditions including well production rates, fluid properties, and completion sizes using transient 1D flow simulation and more detailed 3D computational fluid dynamics modelling. The results will be used to design future laboratory experiments to assess the effect of these stresses on the integrity and effectiveness of the iron sulfide scale in reducing corrosion rates.

Monitoring individual layer performance (in commingled completions) and integrating the results into an overall understanding of field performance has always been a challenge. Overcoming this challenge allows for a better, layer-by-layer, understanding of the reservoir, and therefore will have a significant impact on well and reservoir management. This paper presents a success case for the integration of a novel approach, and specific applications are highlighted. The Khuff Formation of the North Field is a multi-layered carbonate reservoir formed from four main reservoirs; K1, K2, K3 and K4. As most of the RasGas wells are completed commingled through the main Khuff reservoirs, a hybrid approach for integrating multi-layer pressure transient analysis (PTA) with production logging tools (PLT) analysis of flow and pressure profiles was developed. The process also accounts for the physics of carbonate matrix acidisation. The outcome of this technique has helped RasGas better assess the stimulation effectiveness in commingled wells, and establish baseline performance for individual layers. This work was used to build improved inputs for reservoir simulation models, thus providing more accurate predictions of reservoir performance. Additional benefits of this technique have been: 1) identification of wells with impaired productivity (candidates wells for re-stimulation), 2) a better understanding of the stim jobs (areas for improvement for future jobs), and 3) an improved understanding of log kh vs test kh variation (to understand specific questions on well performance). This paper discusses the outcomes, applications, and the added value of this integrated methodology. The paper will also demonstrate examples where the technology was applied and the value that was added to the ongoing surveillance and future development activities.