Lost circulation is one of the most common problems encountered during drilling operations. Investigation of various conditions that lead to lost circulation makes it possible to modify drilling parameters to minimize the likelihood of this problem's occurrence. In this paper, a statistical evaluation model was constructed for wells drilled in Southern Iraq. This model was conducted to obtain a deeper understanding of the relations between the operational drilling parameters and mud properties with the risk level corresponds to the loss circulation rate. Data and pertinent information of lost circulation events were collected to initiate a database of wells drilled in Southern Iraq in different fields. Statistical and sensitivity analyses were applied to identify the most effective parameters on the loss circulation rate. Results of this work show that the flow rate (FR), rate of penetration (ROP), mud weight (MW), Fluid loss (FL), marsh funnel viscosity (MFV), and well inclination (INC) strongly affects the loss circulation rate. While, weight on bit (WOB), wellbore azimuth (AZI) and inclination [INC]), plastic viscosity (PV), yield point (YP), 10-s gel strength (Gel 10"), 10-min gel strength (Gel 10'), and solids content (SC) were found to have a lesser impact on the loss circulation rate. The use of the statistical analyses could assist the drilling engineer in estimating the risk of lost circulation occurrence during well preplanning and developmental phases.

1. INTRODUCTION

Lost circulation, also known as lost returns, stands for the undesirable loss of the whole or a part of drilling fluid into a formation (Toreifi et al., 2014). Lost circulation has consisted as a detrimental problem because it leads to severe safety hazards such as wellbore stability, pipe sticking, well control, and damage in formation pores by plugging them (Abbas et al., 2019a; Dahab et al., 2020; Asmaa et al., 2020). Historical evidence shows that the lost circulation occurs during drilling on approximately 20 to 25% of wells drilled worldwide (Economides 1998). Such a problem can potentially cause the petroleum industry to spend about US 800 million per year (Agin et al., 2018). Consequently, it becomes the focal point for drilling engineers in the last decade.

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