Technique Overcomes Scarcity of Solubility Data in Scale Prediction for HP/HT Conditions
Authors: Carpenter Chris 
Published: February 2020

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 193564, “Scale Prediction and Mineral Solubility Under HP/HT Conditions,” by David Nichols, Neil Goodwin, and Gordon Graham, SPE, Scaled Solutions, et al., prepared for the 2019 SPE International Conference on Oilfield Chemistry, Galveston, Texas, 8–9 April. The paper has not been peer reviewed.

Accurate scale-prediction modeling is only possible when reliable mineral solubility data are available under the required conditions. It is recognized that the relative paucity of high-pressure/high-temperature (HP/HT) solubility data can result in inaccurate predictions because current models extrapolate from data obtained under more conventional conditions. The complete paper describes the generation of additional fundamental solubility data under HP/HT conditions and comparison of the obtained values with several existing models.

Introduction

The limited availability of HP/HT mineral solubility data in simple brine conditions and the paucity of solubility data in complex brine systems means that existing scale-prediction models, including those used in conventional software in common industry use, are forced to extrapolate, resulting in inaccurate and potentially misleading predictions.

Efforts have been made in recent years to address this issue by generating additional solubility data under HP/HT conditions to allow improved models to be developed. It is apparent from the reported work, and comparisons with previous literature, that these measurements are not trivial and require detailed understanding, development, and validation to ensure that reliable measurements are obtained. However, full details, comparison, and verification of the various experimental methods used are rarely presented and discussed, particularly in older works. The experimental method used must demonstrate that equilibrium solubility has been reached, preferably in a reasonable laboratory time period. The complete paper discusses the experimental considerations that must be taken into account when generating HP/HT solubilities, and presents methods to confirm that true equilibrium conditions have been reached.

Experimental Methods

Solubility measurements were made using a purpose-built laboratory test rig capable of attaining 250°C and 30,000 psi, though data described here were obtained between 50 and 200°C and between 10,000 and 19,000 psi. The design and validation of this rig against literature data has been previously described in detail in paper SPE 184568, and is only presented in summary in the complete paper. A schematic of the apparatus is shown in Fig. 1, while Fig. 2 shows the pressure vessel used in the system.

Results

There are two primary experimental approaches that can be applied, and both have been investigated in the course of this work, using the same combined flow-through and static method. The first is to approach equilibrium from below by dissolving the test mineral into the brine composition of interest. In the second, the precipitation approach, a mixture of two fluids, one containing the relevant cation and one the corresponding anion, are mixed to create a solution oversaturated slightly in the test mineral before being applied to the sample. This seeding is intended to result in crystallization of the excess solute, relieving the supersaturation and resulting in a solution containing equilibrium concentration of the mineral under the chosen conditions. However, the rate of the dissolution and precipitation processes can vary considerably for a given mineral and, if this is not recognized, inaccurate solubility measurements will result.