RIASSUNTO
Abstract
Gravel packing can be successfully performed on high-angle wells (70°-80°) when special consideration is given to the completion design and details of field procedures and practices. Through analysis and modification of current techniques and research done on full scale models, Chevron experimented with and developed several techniques to successfully gravel pack high-angle wells.
The high-angle gravel packs were performed utilizing the following criteria: solids free completion fluids, low-side perforating, modified screens, combination tools (circulating pack), maximum tailpipe (washpipe)-liner ratios, medium viscosity carrier fluids, maximum pumping rates, and special attention to quality control.
Sand-out pressures, subsequent pack tests, quantity of gravel placed (percent packed volume) and extended flow tests of up to 20 MMCFD have proven the success of the design and procedures implemented.
Introduction
In late 1979, Chevron set a massive 785' tall structure in 685' of water at Garden Banks Block 236 in the Gulf of Mexico. Chevron planned to drill up to 20 wells to develop four blocks (192, 193, 236 and 237) from this one platform. Due to the shallow depth of the production sand (±4500' SS) and the extensive faulting in the area, almost all of the wells required high angles, (most in excess of 70° from vertical) to obtain horizontal displacements from the structure in excess of 5000' (Figures 1 and 2). The formations near the surface were very soft and in order to get a competent seat for the drive pipe, drive shoe penetrations of 150' were the norm. As a result, kick-off points for these wells were at least 1000' below the rotary. In order to reach these extremely high angles, rates of build in the angle building portions of the hole varied from four to six degrees per one-hundred feet.
Due to the shallowness of the sand, most of the formations drilled were pressured clays and shales (gumbo). These formations were cased off with 9-5/8 casing prior to drilling into the production sand. After drilling the sand, a 7"" liner was set through the productive interval. The wells were then suspended to be completed at the end of the drilling program.
Existence of unconsolidated â??flowingâ?¿ sands, log evaluation and sidewall core analysis indicated that sand control would be required to effectively produce these wells. Basically, there are three methods used to control sand production: 1) rate control-development of natural sand arches, 2) mechanical sand retention by screens, liner and/or gravel packs, and 3) consolidation of the individual grains in place.1,2
Mechanical retention by screen and gravel (gravel pack) was chosen over the other two methods based upon: 1) length of intervals being completed, 2) economics, 3) chance for success (simplest, most consistently reliable approach), and 4) past experience and performance. Sand consolidation is generally restricted to a short interval of less than ten feet, zones with limited sand producing tendency and high reservoir pressure.3 In addition, it was believed that a producing rate that would not cause sand production, if it existed, would be uneconomical.
Design, Processes and Equipment
When planning the completion of any well, special consideration has to be given to completion design and the procedural sequence by which it will be performed. This is especially true when gravel packing unconsolidated sands in high angle wells. The discussion of design and procedural sequence will be as follows:
Gravel Packing Parameters
Gravel Sizing and Quality
Screen Sizing and Design
Pre-Pack Well Preparation
Perforating for Production
Perforation Clean-Up
Installation of Gravel Pack Assembly