Continuous Scratch Testing on Core Allows Effective Calibration of Log-Derived Mechanical Properties for Use in Sanding Prediction Evaluation

Abstract

This paper discusses the use of continuous scratch testing for evaluation of rock heterogeneity and effective correction and calibration of mechanical properties predictions using wire line logging. Understanding the vertical variability of rock properties (strength and moduli) along the reservoir thickness is of primordial importance for sanding prediction analysis. Logging measurements (GR, Sonic, Density and Porosity) are traditionally used to identify troublesome intervals, to select sections for laboratory testing and for extrapolating laboratory data to untested reservoir sections. However, depending on the scale of the reservoir heterogeneity, the log measurements, by providing integrated values over their window of vertical resolution, can considerably underestimate the strong reservoir sections and overestimate the weak reservoir sections. This leads to misrepresentation of critical weak layers with high potential for sanding and potentially to large errors in the completion strategy with costly long-term consequences. Corrections are thus necessary and are traditionally based on simple unconfined compression (UCS) tests conducted sparsely or at high density, varying from operator to operator. This paper presents a method for log-correction based on continuous core scratching in the laboratory and shows that the methodology is effective in evaluating changes in properties with high accuracy and a resolution of at least 1 cm. Introduction Rock mechanical properties are fundamental to analysis of wellbore stability, sanding potential, mud weight requirements, casing collapse, compaction potential and many other problems related with the short- and long term mechanical stability of the well and the reservoir. These predictions, when correct, may help saving millions of dollars in drilling and completions costs, and may allow preventing long term and costly consequences. A particular problem in petroleum geosciences is that rock mechanics data is hard to come by and limited to small sections of the reservoir. The lack of data and, when available, the lack of significant volume representation of these data, has frustrated many investigators and has led them to pursue alternative avenues (e.g., correlations, statistical estimates) to obtain rock mechanical properties. In petroleum-related rock mechanics, computational capabilities for numerical simulations of wellbore stability, sanding potential, reservoir compaction, casing failure during production and other pressing mechanical problems have evolved into sophisticated tools whose demands for field data (e.g., in-situ stress, rock properties, coupled hydraulic-thermal-mechanical properties, and the lateral and vertical variability of these properties across a reservoir) far exceeds the industry's capacity to provide it. Thus engineers struggle to develop alternative sources of data via correlations, inferences from seismic prospecting, inferences from log measurements, or by developing knowledge-based criteria to guess reasonable values. However, lacking good quality data, rock mechanics predictions are weak and most importantly, the industry confidence for implementing them to field applications is low. Thus rock mechanics analysis, limited by lack of data, is a service that does not fulfill its potential for minimizing well construction costs or preventing, long term, production-related problems.

Evaluation of rock mechanical properties via scratch testing and its impact on energy production: comprehensive review

“…TerraTek TSI TM [60,61,62] is another type of scratch test equipment utilized for estimating rock mechanical properties and other parameters through the scratch testing method. Its system consists of a moving cart containing a core sample holder, a cutting tool with the rock "scratching" capability, loading cell to measure the vertical and horizontal forces acting on the rock surface.…”

An Experimental Study to Reduce the Fracture Pressure of High Strength Rocks Using a Novel Thermochemical Fracturing Approach

“…The ductile mode takes place at a shallow depth of cut and is associated with plastic flow, while the brittle mode occurs above a threshold depth of cut and is characterized by the propagation of tensile crack. In the ductile mode, experiments with a sharp cutter show that the energy required to remove a unit volume of rock, referred to as the intrinsic specific energy ε, is well correlated with the uniaxial compressive strength [45,46]. The width of the cutter used was 10 mm.…”

Squeeze Chemical for HT Applications—Have We Discarded Promising Products by Performing Unrepresentative Thermal Aging Tests?

“…• The final oil saturation (S or ) was estimated to be 0.39, which gave an effective pore volume of 70.9ml available to the brine phase • 5pv of 10% scale inhibitor in 6% NaCl with 200ppm oxygen scavenger (stock solution) was injected at 1ml/min • The core was then shut-in for 55 days together with a number of sealed samples (ampoules) of the stock solution which were later used as references, see Figure 4 • A spacer of 1.1pv 6% NaCl without oxygen scavenger was injected at the start of the desorption stage, see explanation in the next section • Desorption continued with 4pv of Brine A before the core was shut-in again • The multiple shut-in/desorption continued with the last sample collected after 110 days of aging, equivalent to 220pv post flush • The permeability of the core was checked to ensure no formation damage • The rig was then cooled down and the core was visually inspected • Due to the apparent disintegration of the rock matrix at the inlet face, scratch test for strength determination [11,12,13] was carried out along the length of the core • ESEM images of the core were also taken to check the conditions of the core…”

“…After dismounting at the end of the experiment the core was cut in four sections as shown in Figure 19. Sections 1, 2 and 3 were sent for strength analyses using the scratch test [11,12,13] . Figure 20 shows the strength of the Berea core as a function of distance from the front.…”