Horizontal Wells Drilling in Vertical Lateral Beddings Successfully Tap the Upside Remaining Oil in Heavy Flooded Sandy Zones

Abstract PI reservoir, the main reservoir of Daqing oilfield has an average thickness of 15–25 metres with one-third OOIP of the total. This reservoir is highly flooded that their daily oil productivity is 4–5 tons /d per well, with a water-cut of 95%. But the current recovery degree of OOIP is only about 40 %, and one-third of the top thickness has not been tapped. Injected water circulating along the big channels can not further enlarge swept volume. In recent years, we drilled horizontal wells to recover the top remaining oil in the reservoir. Since muddy intercalation seldom develop within channel sand, this often results in fully flooded in horizontal wells by fast water breakthrough from the bottom to the topside of the reservoir as top remaining oil is produced. This paper describes reservoir with geological model, and shows the internal architecture in the reservoir accurately. We use lateral muddy intercalations as barrier beds to prevent vertical flood along the wellbore, Once flood occurs, we perform water-plugging in specific lateral beddings. Let's take well N1-H25 for example, it was designed as a horizontal well with vertical lateral beddings. It was brought into production in Dec., 2004, initially it produced 123 tons per day, which was 22 times the production of any well in the same formation. In July 2005, its daily oil production was 81 tons, with a low water-cut of 28.9%. It has accumulated 26000 tons of oil by Dec. 2005. This well was designed to pass 8 lateral beddings, but in fact, it passes 16 lateral beddings. Its lateral muddy intercalation successfully prevented water flood along the wellbore, and effectively produced potential remaining oil in top highly flooded sand reservoirs. Preface PI reservoir dominate most of the OOIP of Dazing oilfield, it accounts for one-third of the total. Geologically, these zones were flood plain deposited sandstone of fluvial-delta (Fig1), its average effective thickness is more than 6 meters, with a permeability more than 1 Darcy and a good horizontal connectivity. Most of those zones are positive rhythm, coarse sandstones dominate the bottom part of the zones, and permeability is favorable; but at the top part, sandstone particle is much small, with an adverse permeability. In this kind of zones, it is prone to occur water breakthrough along the bottom part, while the top is not flooded as how we want. The PI reservoir was developed with water-flooding in 1960; wells were arranged in rows, with a well spacing of 500X500m. With 40 years of development, 38% of the OOIP has been recovering so far, with a water cut of 93%. In part of the polymer flooding, production degree had reached about 50%; but there is still half of the OOIP untapped. Both core data and numerical simulation show that when water-cut reaches 97% in polymer flooded well, the production degree of channel sand is only 50.63%. Top remaining oil accounts for 31% OOIP of channel sand (Fig.2). Since physical property difference varies greatly in these formations, it is difficult to enlarge conformance volume by means of water flooding. The most effectively way is performing horizontal drilling to produce top remaining oil. Its effectiveness relies on if there is a large areal developed muddy intercalation as barrier layer preventing water breakthrough from the bottom of the zone. 1. Internal architecture study clarifying intercalation development of channel sands We started study on reservoir internal architecture in 2000, which focus in intercalation development. Horizontal intercalation exists in braided river deposition, since interfluvial muddy intercalations consisting of fine particles were seldom retained because of channel sands cut and upgraded each other, it resulted in reservoir characteristics of significant thickness, rough particle, and less barrier layers with no development regularity, it is difficult for staying within. Vertical accretion dominates the internal intercalation distribution in sedimentary units. Observing from the cutaway view, it can be seen that a number of lens shaped channel units locating at random horizontally and vertically (Fig.3-a), and uncertain distribution of muddy intercalation. It is difficult for these intercalations to prevent water efficiently from breaking through at the bottom of the reservoir (Fig.3-b), but there is more lateral accretion intercalation in it.