Evaluation of Contaminated Cement Returns: A Case Study
Authors: Nanda Jajati Keshari , Patil Anil Rangarav , Pal Omprakash Ramkhilavan 
Published: May 2016

Abstract
Mixing of cements of various batches, aging, and contamination impact cement slurry performance. For effective cementing, the cement slurry should completely displace the spacer and/or drilling fluid. Most often, this is difficult to achieve, and a portion of drilling fluid remains in the wellbore and contaminates the cement slurry, which adversely affects cement performance.
This paper discusses a case of cement slurry contaminated by drilling mud. Analysis of the cement samples of all of the batches used during the cementing operation were determined to be suitable in composition and without contamination. However, because of downhole conditions, poor performance of the cement slurry (prepared from same batches of cement) presented a challenge. A study of the contaminated cement samples collected from different depths of the well revealed the presence of drilling fluid chemicals along with cement. A systematic and detailed analysis of the well returns and original cements were analyzed using modern analytical techniques before final conclusions were made.
Based on the study and results obtained, a remedial action was initiated by field engineers to remove all traces of drilling material contamination in the wellbore to help ensure appropriate cementing.

Introduction
Drilling mud contaminating cement slurry has long been considered a cause of failure during oilwell cementing (Morgan and Dumbauld 1952). The spacer and cement slurry should displace the drilling mud from the annulus between the casing and walls of the borehole; it is probable that some mixing of the cement slurry and drilling mud occurs, even under the most ideal displacement conditions. The contamination is believed to occur mostly on the integration of cement-formation interface. When mixed with treated drilling muds, cement can be affected adversely by two factors: dilution and contamination. Dilution is an increase in the water-cement ratio derived from the aqueous phase of the mud. Contamination is the mixing of mud additives with cement (Beach and Goins 1957).
The basic functions of oilwell cementing are to provide zonal isolation, structural support for the wellbore, and casing protection against corrosive fluids. An unsuccessful cement job will cause failure of cement to perform these functions. Wellbore cement problems are grouped into two categories: short- and long-term.

Short-Term Cement Problems.
The following are short-term cement problems.

Incomplete Cementing Attributed to Casing Eccentricity.
This occurs when the casing is not properly centered in the wellbore, causing preferential flow of the injected cement through the wider section of the casing-formation annulus. This results in the narrower side of the annulus not being cemented.

Inadequate Drilling Mud Removal.
Gelled mud or mud cake in the wellbore can be difficult to remove, leaving residual mud on the wellbore wall. The residual mud interferes with the bonding of cement to the formation, this can result in a weak cement formation bond.

Influx of Formation Fluids During Cement Setting.
During the transition from cement slurry to hardened cement, the cement loses some of its ability to transmit hydrostatic pressure; this might allow gas or reservoir fluids to flow into the wellbore from the formation.

Long-Term Cement Problems.
The following are long-term cement problems.