Advancing Isolation Techniques for Geothermal Wells: Development of Polymer and Nanoparticle System
Authors: Shehata A. M. , Kalia N. , Comer R. 
Published: March 2024

Geothermal energy has emerged as a promising renewable energy source with its continuous availability and independence from weather conditions. However, the development of geothermal wells faces various challenges that limit its widespread adoption. One of these challenges is the decline in the efficiency of geothermal wells over time due to the reduction in permeability and scale formation. Acidizing is a common technique used to enhance productivity of the wells. During the stimulation of these geothermal wells, there is a need to temporarily block the high permeability zones for sufficient duration to selectively direct the injected acid to target formation.

This study introduces a novel approach for the temporary isolation of high-permeability zones in geothermal wells using a specially formulated diversion fluid. This fluid is a mixture of a polymer solution with nanoparticles and crosslinking agents. The study primarily explores the efficiency of hydrolyzed polyacrylamide (HPAM) polymer solutions at various concentrations (5,000 – 30,000 ppm) and nanoparticle levels (100 – 3,000 ppm). The goal is to determine the optimal concentration for temperatures ranging from 100 to 250°C, examining different polymer, nanoparticle, and crosslinker combinations.

Rheology measurements and static tests were conducted to assess the behavior of the prepared fluid both before and after heating. The viscosity of the solution was examined versus both time and shear rate at a wide temperature range of 25 to 120°C. Static tests were performed to evaluate the thermal stability of the solutions, as well as gelation time and the time required for the gel to completely break down. These tests were conducted at various temperatures ranging from 100 to 250°C. The objective was to assess the effectiveness of the formed gel plug in temporarily blocking the flow in addition to the gel’s ability to return to a liquid state within three to five days.

The results obtained from this study showed successful development of an innovative fluid for temporary isolation, effective up to 200 - 250°C. This achievement was made possible by combining nanoparticles with hydrolyzed polyacrylamide polymer solutions after adding metal-based chromium, resulting in a physically crosslinked gel due to complexation between the metal and carboxylic acid groups on HPAM. Importantly, the gel plug can revert to a liquid state under reservoir conditions, eliminating the need for external breaker chemicals. This feature simplifies the removal of the gel plug post its isolation function. The unique aspect of this study is addressing the problem of early crosslinking at high temperatures, by introducing metal lactate CLD-Z as a crosslinker delayer agent. Five different types of HPAM polymers were tested in this project and it was found that the polymer (A) exhibited a viscosity below 100 centipoise (cp) at ambient conditions and 300 rpm with concentrations up to 15,000 ppm, whereas polymer (B) initially had a viscosity below 180 cp, which reduced to 70 cp when using 15,000 ppm NaCl. Also, the viscosity of the polymer solution, when combined with the crosslinker, maintained a desirable range of 50 to 200 cp at ambient conditions.

This study demonstrated that the addition of crosslinker delayer agent CLD-Z notably extended the gelation time of the polymer solution, with the extent of this delay being dependent on the concentration used. In particular, with a crosslinker delayer agent CLD-Z concentration of 10 (gpt) at a temperature of 120°C, an average extension of two hours in the gelation time was observed. Additionally, the introduction of crosslinker delayer agent CLD-Z was found to decrease the viscosity of the polymer gel solution. The optimal concentrations and conditions varied based on the target formation temperature and polymer type, resulting in varying isolation durations. Polymer-(A) achieved optimal performance at 150°C with specific concentrations of polymer (15,000 ppm), nanoparticle (250 ppm), crosslinker (5 gpt), and delayer (10 gpt). Polymer (B) was similarly effective at 200°C with comparable concentrations. In static tests using an HPHT cell, conducted under high-pressure (1,000 psi) and high-temperature (200°C) conditions, the polymer fluid exhibited dynamic transformations over a six-day period. Initially, it formed a stable gel phase without any water presence on the first two days. By the sixth day, the fluid had completely converted to water, with no observable gel phase remaining.