Sequential Stimulation Technology -Thermal Lattice™ – To Enhance Connectivity Between Injectors and Producers for Super Hot Reservoir (SHR) Enhanced Geothermal Systems (EGS)
Authors: El-Rabaa W. M. , Abass H. H. , Asher G. B. , Siddiqui N. 
Published: January 2025

This paper presents new stimulation technologies applied in a sequential manner to develop a large network of fractures emanating from the injectors to enhance connectivity/permeability to producers in Enhanced Geothermal System (EGS).

The objective of creating a large fracture conductive network between injectors and producers in EGS is well established. The purpose of the network is to increase exposure of reservoir hot rocks to yield efficient heat harvesting, reduce friction of the injected fluids to reach the producers, and reduce cost of horsepower (HHP).

The stimulation sequence proposed in this paper to enhance connectivity in the EGS, is to start by tensile fracturing followed by pulse fracturing and hydro-shearing, herein is described as "Thermal Lattice™" creation. The main objective of the initial high-rate tensile fracturing is to guide/channel the subsequent low-rate stimulations performed at a pressure below the minimum horizontal stress (Shmin) to facilitate communication with existing natural fractures away from the injectors. The benefits of sequential stimulations can be assessed by comparison of reservoir flow capacity or rock transmissivity (T = kh/µ) measured after each type of stimulation. This calculation is performed on surface pressure fall-off after it is corrected for thermal effects on wellbore fluid based on rock thermal gradient and wellbore fluid properties.

A series of hydraulic fracturing laboratory tests were conducted using volcanic rocks, granite (pink and grey) and basalt to assess the changes in transmissibility post every type of stimulation. The tests were conducted on volcanic rock samples of 5″ × 5″ × 10″ and 12″ × 12″ × 12″. A stress frame with a capacity of 100 klb and temperature up to 400 ºF, was used in testing these samples. Upon completing the test, each sample was cut open to view the induced fracture-network geometry. It was observed that a major enhancement of kh/µ, as expected, is generated after the first tensile fracturing event, followed by the subsequent below Shmin stimulations of cyclic injections, with up to 8x with multiple cycles of injections. The enhancement of rock transmissivity post tensile fracturing by hydro-shearing alone was minimal.

The paper also presents heat harvesting results from an Integrated Asset Management (IAM) model to show the benefits of the thermal lattice. The model is based on a pressure/enthalpy solution that includes an injection well, thermal lattice, production wells and power plant. The thermal lattice model harvests heat from multiple channels that connect the injector to the producer(s) where channels that do not connect, will not contribute to heat harvesting. It is planned to demonstrate the concept in the Super-Hot Rock (SHR) volcanic reservoir operated by Mazama at Newberry, Oregon which is a super-hot reservoir at temperatures &gt; 375 ºC. The Heat Harvester, as an IAM model, describes a case at 400 ºC.

The brittle-ductile behavior of Newberry reservoir is assessed based on its mineralogy and published data of similar rock characteristics. Also, its stress regime is calculated and presented in the paper based on wellbore breakouts observed along 400 ft in one of the explorations well, 55-29.