Dual Openhole Gravel Pack in Shaly Fine Sands

Abstract

Summary This paper discusses the design and execution of dual-zone gravel packs in very shaly and silty formations of median sand grain size less than 30 um. An oversized gravel was selected for sand control and, as consequence, the openhole gravel-pack technique was adopted to reduce the effect of intermixing between formation and gravel-pack sand. During the completions in the first five wells, we encountered and solved several operational problems, including those involving (1) borehole stability, (2) setting of inflatable packer for zone isolation, (3) hole preparation, and (4) gravel placement. From the lessons we learned, another 11 wells were completed with dual openhole gravelpacks without significant problems. After 1 year of production, the flow performance from the wells met or exceeded the initial objectives. Introduction The Giovanna field is a gas-bearing reservoir situated in the offshore Adriatic area (Fig. 1). It is a multiplayer reservoir comprising 30producing intervals that consist of thin interbedded layers of partially unconsolidated sand and shale. The total pay zone has a thickness of more than700 m and is located at depths between 1200 and 2000 m. As a result of the small grain size of the sand and the high clay content, the reservoir rock has a very low effective gas permeability (10 to 30 md). To develop this reservoir, it was necessary to make provisions for a completion that would prevent the movement of the incompetent sands. Because of the low formation permeability, sand consolidation was immediately rejected as a suitable technique for providing sand control and consideration was given to mechanical methods (gravel packing). In many cases, the inside-casing gravel pack (ICGP) introduces a large pressure drop across the completion,1 with a consequent reduction in the production capacity and performance of the well. This result is especially true when, as in this case, a very small size of gravel (100 to 120 U.S. mesh)is required. On the other hand, the openhole gravel pack (OHGP) generally is not recommended in the presence of clays because there is a high probability of intermixing with gravel. After careful analysis of the advantages and disadvantages of each of these techniques, we decided provisionally to complete all 16 wells with OHGP's in dual parallel completion. With only 16 wells available to develop the reservoir, it was necessary to combine the 30 producing intervals into 11 producing pools. This decision was based on homogeneous pore pressure and depletion of the commingled producing intervals. As a result, it was necessary to complete long intervals (40 to 60m) for the exploitation of the gas resources. Because no previous experience has been gained with gravel-pack completions in very fine and silty sand, we decided to proceed with the following course of action.Perform a sand-flow test (SFT) to verify that sand production would occur.Execute a dual OHGP in the first well to ascertain the feasibility and efficiency of this type of completion. Furthermore, a single ICGP was executed at a later stage and confirmed that the OHGP was the most appropriate completion for the Giovanna field. This paper will illustrate the problems encountered, the remedies introduced, and the results obtained from these completions, with particular reference to (1) evaluation of the risk of sand production, (2) gravel-size selection, (3) borehole stability, (4) zone isolation, (5) hole preparation,(6) gravel placement, and (7) flow efficiency.

Screenless Completions: Cost-Effective Sand Control in the Adriatic Sea

“…The screenless completion was performed rigless, yielding substantial savings over a rig-based workover. Production from the zone following the screenless completion was 2 1 2 times that of the initial production from the zone, normalized to the same production interval. Following the cleanup period, no sand or proppant was produced during the 2 1 2 months of production, even though formation water was produced.…”

“…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 10͒ were also installed in two sublayers ͑pools͒ 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.…”

Screenless Completions: Cost-Effective Sand Control in the Adriatic Sea

“…All of the original wells in this field were completed with open-hole 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.…”