Sarawak Shell Bhd operates the Shallow Clastics field, which targets two shallow gas-bearing reservoirs, H1 and H2. An early appraisal well had been drilled deviated, and 9–5/8-in. casing was used. A commingled upper completion and a sand-face completion using large-OD expandable sand screens through both targets was used. Upon completion, the well was cleaned up through a temporary well clean-up and test facility. The purpose of the well was to test productivity and evaluate the integrity of the downhole sand-exclusion installation. Fines production, possibly due to the failure of the sand exclusion method, steadily increased to the extent that the well was deemed unproducible to the facilities.
Due to the high fines produced from this first well, a re-evaluation of the sand exclusion method was undertaken before development of the remaining field. The re-evaluation included more extensive core analysis and reviews of the types of sand-face completions suitable for development of the H1/H2 reservoirs. This evaluation enabled the operator and a service/engineering company to develop an innovative sand-exclusion method that combined several new technologies.
This paper discusses the evaluation and the four wells that have been completed to date with this new sand-exclusion method and the well configuration to address the completion needs. The field implementation of the design, key success factors and results will also be summarized.
Introduction
Sarawak Shell's Shallow Clastics field consists primarily of two shallow gas bearing reservoirs, H1 and H2, at approximately 2,650 ft TVD. These reservoirs are laterally extensive, covering an estimated area of 200 square km with estimated gas-in-place (GIP) in excess of 2 Tscf. The reservoirs are made up of a sequence of highly laminated sand and shale deposits with significant sand-size variability and high fines content, Figure 1. The unconsolidated nature and high fines content make downhole sand exclusion mandatory to maintain production.
An early appraisal well was deviated and cased through both the H1 and H2 reservoirs. The well had been completed with 9–5/8-in. casing, expandable sand screens across both reservoirs, and a commingled upper completion. Upon completion, the reservoirs were cleaned up through a temporary well clean-up and test facility to test the productivity and evaluate the integrity of the downhole sand-exclusion method. Fines production, possibly due to the failure of the expandable sand screen, commenced almost immediately and steadily increased to the extent that the well could no longer be produced.
Because of the results in the first appraisal well, the operator initiated a re-evaluation of the field development plan and well concepts. This re-evaluation consisted of additional core sampling and a review of the well types suitable for the H1/H2 reservoirs. The analysis of the core sample from F13–6 showed sand grain sizes of D50 80–220 µm, which were smaller than expected, Figure 2. The Uniformity Coefficient, given by the D40/D90 ratio, varied between 1.5 and 34, which indicated that the formation sands were highly non-uniform. Another evaluation was performed to look at the Sorting Coefficient, given by D10/D95, which varied between 2.5 to 130, giving further proof that the formation sands contained highly nonuniform grain sizes, Table 1. The fines content in these reservoirs reached 34%, which is considered high for attempting to produce with minimal-to-no solids production.
The original wellbore selection consisted of deviated wellbores with stand-alone or expandable screens across the openhole H1 reservoir. Following the well testing results on the appraisal well, a full qualitative analysis of the well type suitable for these reservoirs was completed. The analysis looked in depth at other sand exclusion methods available, including traditional openhole horizontal gravel pack (OHHzGP), openhole horizontal gravel pack with concentric annular packing (CAPS), frac and pack, internal gravel packs (IGP), and expandable sand screens.
