Properly designed temperature history nanoparticles may improve residual oil saturation estimates from SWCT tests

Highlights

• Temperature gradients may influence S_or estimates from SWCT tests.

• We have developed design criteria for temperature history nanoparticles.

• Simulations show that the hypothetical nanoparticles may constrain S_or estimates.

Abstract

Considerable resources are required to develop new nanoparticles for various applications. Before too much effort is put into the development of nanoparticles that might not work properly, it may be wise to spend more resources on studies of how they should de designed to be really useful for solving an identified problem. In particular, we believe that it is a good idea to demonstrate the usefulness of as yet hypothetical nanoparticles by performing computer simulations of the system at hand with them. To exemplify this idea, we use the so-called Single Well Chemical Tracer (SWCT) test, the purpose of which is to estimate the residual oil saturation (S_or) in the vicinity of a wellbore. In the majority of reported SWCT tests, ethyl acetate is pushed into the oil bearing formation from a well and partly converted to ethanol by hydrolysis. The chromatographic separation between the ethyl acetate and ethanol during back production is then used to estimate S_or. The hydrolysis rate of ethyl acetate depends exponentially on temperature, and if a temperature gradient develops across the ethyl acetate bank, this has to be accounted for if a correct S_or estimate shall be obtained. Nanoparticles that are co-injected with the ethyl acetate and that have a ‘memory’ of the temperature history they have experienced may prove useful in this respect. We suggest several design criteria for such nanoparticles. The envisioned nanoparticles encapsulate a dye that degrades when heated as well as an internal standard that does not degrade. This property facilitates easy calibration of the fluorescence signal. We propose a simple rule for deriving degradation rates suitable for the temperatures and times typical for a SWCT test. The nanoparticles move with the ethyl acetate so that their temperature history is representative for its hydrolysis. It is important that it is easy and uncostly to analyze the temperature history nanoparticles. They have to satisfy modern health, safety and the environment (HSE) requirements, and have acceptable production and deployment costs. An axis-symmetrical numerical model is used to demonstrate the usefulness of such designed nanoparticles based on data from a published SWCT test. First, we calculate the temperature history of the nanoparticles using a suite of injection temperatures. The pre-exponential factor and activation energy of the dye derived from the design criteria are then used to simulate the degradation in nanoparticle fluorescence with time for the various nanoparticle temperature histories. From these data, we calculate the aggregate average and standard deviation of the nanoparticles' fluorescence in the produced fluid, i.e., information that would be available if the nanoparticles had been used in a real SWCT test. We find that such data provide useful constraints on the temperature history and thus helps us pin-point the correct S_or value. The temperature history sensitive nanoparticles may potentially also yield valuable information on the effective thickness and other characteristics of the target formation, the surrounding rock and perhaps even on the wellbore and its thermal interaction with the reservoir rocks.