Stability of Gravel-Packing Materials for Thermal Wells
Authors: Underdown D.R. , Das K. 
Published: December 2007

Several different types of materials have been evaluated for use in gravel packing high-temperature thermal wells. The materials include Ottawa sand, sand from Venezuela, resin-coated sand, sintered bauxite. and high-alumina beads. The materials were evaluated at various temperatums up to 600 deg. F (316 deg. C] and pH's ranging from 7.0 to 11.0. The tests were conducted by passing water in the fluid phase through samples of the various materials at the desired pH and temperature. Weight-loss measurements and visual inspection by a scanning electron microscope (SEM) were performed on each sample to determine the effect of the various temperatures and pH's.

The steam evaluation of the various gravel-packing materials shows that the alumina-based materials are best suited for gravel packing thermal wells, where the temperature may reach 600 deg. F 1316 deg. C1 and the pH of the aqueous phase of the steam reaches 11.0. The alumina materials have very little weight loss and maintain a uniform shape. Quartz materials show dramatic weight losses as temperature and pH increase. The weight loss of resincoated sand is most affected by increasing pH al elevated temperatures.

Introduction

Steamflooding and stimulation by steam are two of the most widely used methods for the production of heavy crudes. Most thermal recovery operations are in formations that are classified as unconsolidated and that require some form of sand control. One accepted method of sand control is gravel packing the well with a highquality silica sand or other suitable materials, such as sintered bauxite or resin-coated sand.

Shallow wells generally are stimulated with steam injection at moderate rates and temperatures under 450 deg. F [232 deg. C]. Deeper wells, however, may require steam at temperatures higher than 600 deg. F [316 deg. C to be injected at the wellhead. This high-temperature steam is required to ensure that enough heat is transmitted to the formation in the deep wells to allow for efficient recovery of the heavy crudes. It is well established that the solubility of quartz sand and other siliceous materials in water increases sharply at high temperatures and pH's (see Fig. 1). Because of the composition of certain feedwaters used for steam generation, the liquid phase of the injected steam can have a relatively high pH. Where high pH and high temperatures are observed, gravel packs composed of quartz sand can be relatively short-lived.

Steam Evaluation of Gravel-Pack Materials

Several different types of common gravel-pack materials were evaluated. Two sources of gravel-pack sand were tested to determine whether differences in quartz content were important to the stability of quartz at elevated pH and temperature. Ottawa sand, which contains 99.9% quartz, and a sand from Venezuela, which contains 95 % quartz and various amounts of impurities (such as feldspar goethic, kaolinite, chlorite, and illite), were as subjected to various temperatures up to 600 deg. F [316 deg. C and pH's up to 11.0.

Two types of bauxitic materials were chosen for evaluation. The first material was the sintered bauxite commonly used as a high-strength fracture proppant. The second sintered-bauxite-type material is a byproduct of a manufacturing process and contains more A1203 than the more common fracture proppant. Table 1 shows a comparison of the properties of the two sintered materials. Sintered bauxite and high-alumina beads are very stable in high-temperature/dry-heat environments, as shown in Table 2. Terns at thermal-well conditions of temperature and pH were performed to determine whether the differences in the amount of A1203 of the two bauxitic materials would result in greater stability.

The use of resin-coated sand to control sand in thermal wells has been growing in popularity over the past several years. Improvements in resin chemistry and manufacturing processes have helped to increase the thermal stability of this type of material and to prolong its effectiveness in the harsh environment of steam-recovery wells. Tests were performed to determine the stability of one such material as a function of pH and temperature.

Effect of Feedwater

Many of the waters used to produce steam for thermal recovery projects contain HCO-3. This material is found in the formation feedwater and is not removed during the softening treatment before injection into the steam generator. Thermal degradation of the HCO - ions in the feed-water results in CO2 and OH - . Because most steam generators in the field produce 80% steam, the OH ions remain dissolved in the aqueous phase of the steam and produce a very alkaline solution depending on the concentration of HCO - in the feedwater.

JPT

P. 2006^