Remedial Sand-Control in Mature Field: Results & Lessons Learnt from through-Tubing Sand Screens Operations in the Gulf of Guinea

Abstract

The field in which took place the remedial intervention discussed in this article is located offshore West-Africa, in the gulf of Guinea. Oil is produced from unconsolidated sandstone formations of Miocene/Pliocene reservoirs. Currently, out of 360 wells drilled in the field, about 140 wells are still in production and nearly all of them are equipped with sand-control. Historically, the wells were completed as vertical or slightly deviated Internal Gravel-Packs (IGP), Open-Hole Gravel Pack (OHGP) with milled-windows or with Pre-Pack screens, then since 2001 as Frac-Pack and High Rate Water-Pack (HRWP).With the first development dating back from 1978 (over 30 years of production) and 50% of the producing wells older than 20 years, this asset is clearly a mature field. Therefore, the durability of sand control systems has been dearly challenged and on some wells, failure of the sand-control has occurred before the production had been exhausted. This paper will present several rig-less operations that have been performed for sand-control remediation in this mature offshore field environment and the results obtained.We will first discuss possible scenarios of failure of the existing sand-control completions and then present the choices that have been made for the sand-control remediation, in particular the use of premium through-tubing sand screens with no gravel placement and the use of swell packers for the screen anchoring rather than mechanical devices.We will then present the productivity and longevity achieved with such equipment and will demonstrate that even in some unorthodox cases, as can be often found in mature fields, remedial sand-control operations can still prove successful. Practical aspects that are important when considering such operations will also be covered.Finally, recommendations and lessons-learnt derived from successes and failures will be presented. Description of the SituationThe reservoirs produced are buried between 1200 and 1500m in TVD (water depth ranges from 5 to 35m). The sandstone formations are unconsolidated to poorly consolidated (slowness are in general > 120 µs/ft). Permeabilities vary from a few hundred milidarcies to ~2 Darcies. The sands encountered are fine to very fine with D 50 between 100µm and 250µm. X-ray diffractions indicate a very high clay fraction which is often found above 25-30% and that is confirmed by solubilities in Mud-Acid (HCl-HF) of the same range (Table 1). The majority of the clays are of migratory type (mainly Kaolinite, some Illite) and the formations often exhibit high sensitivity to fresh water. During coreflood water sensitivity tests, fines mobilization is hence frequently observed; sometimes to a point where the term "fluidization of the formation" was mentioned in laboratory reports.In view of the formation characteristics, all producers have had to be equipped with sand-control equipment regardless of which reservoir has been completed. As the Frac-Pack and HRWP techniques were introduced only around 2001, i.e. 20 years after the first develo...