Many gas reservoirs in the central Adriatic Sea, offshore Italy, are formed of laminated, low-permeability, dirty sandstones, with as much as 50% clay content. Initially, wells were completed with gravel packs, but the nature of the formations caused productivity to decrease in some gas fields because fines plugged the gravel packs. In the past few years fracpacks have been used to alleviate the fines migration problem. However, there are many gravel-packed wells which are either sanded up and shut in or underperforming because fines have plugged the gravel-pack screens. These wells do not currently justify the expense of an immediate full rig workover.
A candidate zone was selected to evaluate the viability of a screenless completion technique for competent sand control. We placed a highly conductive tip-screenout fracture in the zone, using recent innovations in fracturing fluid technology and proppant flowback control additives, to eliminate sand production. Successful implementation of this technique was confirmed by post-treatment production more than 2.5 times the normalized rate of the initial gravel-pack completion. No formation sand or proppant was produced back to surface during production. The screenless completion technique provided a cost-effective and reliable rigless rehabilitation technique.
Several gas reservoirs in the Adriatic Sea are formed of low-permeability, dirty sandstones; the Giovanna field, located in the Adriatic Sea (Fig. 1), is one example. Its reservoir is composed of turbiditic sediments interbedded with sand, silt, and clay. The average formation permeability to gas is 12 md, with clay content close to, and frequently more than, 50%. All of the original wells in this field were completed with openhole gravel packs; one inside-casing completion and one conventional completion (Giovanna 6, pool 10) were also installed in two sublayers (pools) to assess the productivity and longevity of these kinds of completions in very dirty sands.1 Many different sandstone streaks, more and more often encountered by wells in the Adriatic Sea, such as in the Giovanna field, are grouped and completed in pools.
The Giovanna 6 well was completed with 2 3/8-in. dual-string completion technology. The short string was extended in front of pool 10 by a perforated 2 3/8-in. extension pipe inside the casing. The well was put on production in December 1992, sanded out completely at the end of 1994, and consequently shut-in.
We selected the latest innovative viscoelastic surfactant fracturing fluid technology for field trial in the Giovanna 6 well, pool 10, and the operator decided to use the upper zone of pool 10 as a test for a rigless, screenless sand-control completion technique. Successful application would confirm that the screenless completion sand-control technique could be applied to other wells throughout the area, while failure could be remedied in any similar future application. Screenless sand-control completion technology is of particular interest for dual-string completions in the Adriatic Sea. It will allow low-cost development of any numbers of layers to be produced by the short string, which otherwise could not be drained using conventional sand-control technologies without expensive recompletion. Effective low-cost screenless completion technology will considerably reshape gravel-pack completion strategies.
Several authors have discussed the use of fracturing to prevent the production of unstable formations. Bale et al.2 applied the indirect vertical fracturing technique to produce a weak formation sand free. In this method a stronger interval is perforated adjacent to the weak target interval and a hydraulic fracture is designed to grow into the target zone. Correctly applied this can be a powerful technique but it requires detailed knowledge of the formation lithology and in-situ stresses and the presence of a competent zone next to the target zone.
Kirby et al.3 used 0° phased perforating over a short (30-ft) interval followed by a tip-screenout designed fracture with proppant flowback control to prevent sand production. The objective was to cover all the perforations with the fracture; if the proppant remains in place formation sand cannot enter the wellbore. Resin-coated proppant was used to prevent proppant production. Malone et al.4 have tried a similar technique using 180°C phased perforations in a steamflood application.
The same general idea has been used successfully in over 70 fractures to prevent formation production from horizontal wells in weak chalk formations in the North Sea.5,6 Perforations are shot at the top and bottom of the wellbore over a limited interval (<5 ft) to ensure that the fracture covers all the perforations. Rigorous testing of resin-coated proppants undertaken prior to the treatments ensured that proppant flowback could be controlled.
Fletcher et al.7 discussed the development and application of a technique for predicting the fracture properties required to prevent sand production from out-of-phase perforations. For a given rock strength, the fracture length and conductivity affect the drawdown that can be applied before formation sand is produced from perforations not connected to the fracture. The concept has also been applied by Ortega et al.8 to prevent sand production in deep wells in a high-stress environment.
The reservoir properties for the well are given in Table 1. The initial completion included a perforated extension pipe of the 2 3/8-in. short string, inserted inside 7-in. casing, which was perforated over the entire 190-ft interval at 120° phasing, with deep-penetrating shaped charges. The effective perforation diameter in the 7-in. casing was between 0.2 and 0.3 in. Details of the number and size of the holes in the perforated extension pipe were not available, but we assumed that the holes were between 0.5 and 1.0 in. in diameter. Sand fill was recorded inside the tubing over the entire interval.