Total’s Well R&D teams and the Total Research Center – Qatar (TRC-Q) are currently developing a numerical model, based on extremely precise test results, that will be used from late 2020 for the matrix acidizing operations on wells drilled into carbonate reservoirs.
Improving Productivity Cost-effectively and without compromising on safety
Created from fossilized organisms and multiple paleoenvironments, carbonate reservoirs reflect the full complexity of the processes that formed them. The wide variety of sedimentary facies and the climatic conditions that created them, as well as the successive physical and chemical changes that have altered them over time, result in an extremely heterogeneous geological environment. This is notably characterized at mesoscale by the presence of two distinct media — matrix and fractures — with very different properties and permeabilities.
In these reservoirs, which are primarily made up of limestone and dolomite, pumping acid into the well via the wellhead is the most common technique used to improve flow from the reservoir to the well. One of the main challenges is optimizing the efficiency of these acidizing operations, which play a key role in well productivity and injectivity, and in particular:
- Controlling wormholing, a specific dissolution regime that increases the acid’s penetration depth in the well, with a minimum volume of acid injected.
- Ensuring homogeneous placement of the acid along the length of the wellbores, which are often very long.
The ultimate goal is to sustainably reduce costs — by decreasing the volumes of acid required, simplifying designs and standardizing operations — without compromising on operational safety or well integrity.
Understanding the Mechanisms associated with Acid Stimulation through Experiments
In 2010, Total launched an integrated R&D project at Total’s R&D headquarters in France (CSTJF center in Pau) and Total Research Center - Qatar (TRC-Q) to understand the mechanisms associated with acid stimulation. The aim of the laboratory work being carried out is to improve understanding and control of the physical and chemical processes associated with acidizing treatments, thanks to the acquisition of precise test results. Conducted on different types of carbonate rock (limestone and dolomite), using different acid formulations (Newtonian and non-Newtonian fluids), the program includes:
- Detailed petrophysical characterization of carbonate samples (X-ray tomography and microtomography, X-ray diffraction mineral identification, distribution of pore sizes via mercury intrusion porosimetry, porosity and permeability) and chemical and rheological characterization of the fluids pumped.
- Core flood tests on previously characterized samples at temperature and pressure levels representative of subsurface conditions. These are carried out on test benches, including Qatar’s unique dual core bench, which enables researchers to study fluid-rock interactions in a heterogeneous configuration. Used to test two cores with different mineral compositions and/or permeabilities at the same time, while also optimizing acid placement, this sophisticated test bench provides a very accurate picture, in real time, of the dissolution reaction of each core and more particularly any wormholing effect.
- Analysis and visualization of the X-ray tomography test results and 3D printing of the acid-flooded cores.
Building a Proprietary Model for Predicting Wellbore-scale Acid Stimulation
The data acquired through core flood testing and the characterization of carbonate rocks has contributed to the development of a proprietary 3D acid stimulation model, which solves reactive transport equations in porous carbonate media.
The numerical simulator developed on the model has been validated and has proved its worth at core level, with both Newtonian and non-Newtonian fluids. The scale-up from core to well was initiated in 2019 with the launch of flood tests on large blocks of quarry carbonate in radial flow mode — a configuration that is more representative of certain flows around a wellbore.
The finalized model will offer some significant advantages. In particular, it will enable us to:
- Minimize the number of core flood tests, which are destructive operations that require compliance with stringent HSE standards.
- Simulate scenarios that cannot be tested due to the lack of suitable laboratory equipment, the limited size of the cores or the presence of stimulation parameters that are impossible to reproduce in a laboratory, such as very low acid flow rates or very high temperatures.
- Evaluate the performance of innovative stimulation fluids that are less aggressive and more environmentally responsible.
- The ultimate aim is to create a predictive wellbore-scale tool to optimize the design of acidizing treatments, in terms of both cost and performance, and to analyze the effects of stimulation on wells. It will be used to develop acidizing treatment designs that are particularly efficient and particularly simple, with a view to potentially standardizing operations by region or type of geology (“classic” carbonate, pre-salt carbonate, etc.).
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Total's Research Center (TRC-Q) in Qatar
Carbonate Reservoirs: the Albion Project