The Use of Sidewall Core Analysis in Formation Evaluation

Abstract

Abstract Analytical techniques and procedures which permit accurate measurement of important physical properties and of fluid content of sidewall core samples as received in the laboratory are available. However, hole conditions prior to and during sampling affect the values as measured on the core samples. Also, the impact of the percussion sampler in the sampling process alters some of the physical characteristics of the sample. Comparisons of data on conventional and sidewall core samples and experience have shown the general direction of these effects. Normally, formations along the Gulf Coast have a greater productive capacity than the sidewall core sample data indicate. Water saturations associated with gas, condensate or oil production are greater in sidewall than in conventional core samples. Sidewall core data are valuable as exploratory aids, but data from conventional or wireline cores are generally required for evaluating recoverable reserves, the distribution of reservoir fluids and formation flow characteristics. Sidewall core data usually establish the presence or absence of hydrocarbon content and indicate the probable type of production. Measured permeability and porosity values indicate productive capacity. The data show gas-oil and water-oil contacts. Sidewall sample data are particularly valuable as a basis for "calibrating" electrical log data. They are used to check lithology changes indicated by log data, and they permit evaluation of thin and stray sands. Sidewall core samples probably provide the most reliable data normally obtained on "dirty" or ashy sand zones which show low resistivity on the electrical logs, on sand sections drilled with high-salt-content muds and on shallow bentonitic sands containing fresh water. Consideration of both sidewall core analysis data and electrical log data together increases the value of each. In many instances, it is necessary to consider both types of data in arriving at a correct interpretation. Greatest value can be obtained from the sidewall core data if the analyst has an electrical log of the zone as a guide for general formation characteristics and for zoning the various samples.