Field Results of Shallow Horizontal Drilling in Unconsolidated Sands, Offshore California
Authors: Payne J.D. , Huston Chris , Bunyak M.J. 
Published: October 2008

Four shallow horizontal wells were drilled from Platform "C" in the Dos Cuadras Field, Offshore California to recover reserves previously inaccessible with conventional drilling techniques. The wells had true vertical depths ranging from 746 ft to 989 ft with total horizontal displacements from 1613 ft to 3788 ft. The wells had horizontal displacement to true vertical depth ratios as high as 3.95. The target sands were unconsolidated, high permeability sands. This paper details the planning, drilling, and completion of these wells.

INTRODUCTION

The Dos Cuadras field is a doubly plunging, east-west trending anticline located approximately 7% miles southeast of Santa Barbara, California under OCS leases P-0240 and P-0241. The primary producing horizon is the Repetto formation which is an alternating sandstone and siltstone sequence of early Pliocene age. The top of of shallowest major reservoir, the Brown zone, is less than 300 ft below the mudline. The Brown zone includes two major markers, the CP and DP, and numerous individual markers designated Cl P, C2P, etc. Original oil in place in the Brown zone was 205 MMbbl and recovery to date is 16 MMbbl,

Unocal is the operator of three platforms on OCS lease P-0241, Platforms "A", "B", and "C", set in 190 ft of water. Fig. 1 shows the locations of the platforms relative to the California coast. Commercial production from the field began from Platform "A" in 1968, Platform "B" in 1969, and Platform "C" in 1977. Production peaked at 60,000 BOPD in mid-1 970 and had declined to 6,500 BOPD by mid-1990.

OBJECTIVES

Due to the very shallow producing horizons, these platforms were equipped with rigs capable of drilling with the masts slanted at 30° from vertical. Conventional directional drilling techniques limited completions in the shallowest sands, even with the slant masts, to less than 500 ft from platform center. Fig, 2 shows the limits of completions in the Brown zone, Cl P sand. Because political and economic restrictions preclude the setting of any additional platforms, the horizontal wells were drilled to confirm the ability to economically develop previously inaccessible reserves. Well C-50 was drilled first to demonstrate the viability of the drilling and completion processes, The three subsequent wells verified the results of well C-50 and extended completion limits.

PLANNING OF WELL C-50

Well Location. For the first horizontal well to be drilled in the Dos Cuadras field, the C1 P sand was chosen as the target. This sand is one of the shallowest sands and is one of the thickest and cleanest sands in the Brown zone.

A well course was chosen in a line from Platform "C"to Platform "B" near exploratory well OCS P-0241 #2. This well course, while not the most attractive due to pressure depletion, would provide the best geologic control. Good geologic control was important because the Dos Cuadras field is cut by numerous faults with small displacements. These faults would not greatly affect conventional wells but could place a horizontal wellbore outside of the target sand if intersected unexpectedly.